הֲשִׁיבֵ֨נוּ יְהֹוָ֤ה ׀ אֵלֶ֙יךָ֙ (ונשוב) [וְֽנָשׁ֔וּבָה] חַדֵּ֥שׁ יָמֵ֖ינוּ כְּקֶֽדֶם׃

Take us back, O LORD, to Yourself,And let us come back;Renew our days as of old!

https://www.sefaria.org/Lamentations.5.21

לְמִימֵ֨י אֲבֹֽתֵיכֶ֜ם סַרְתֶּ֤ם מֵֽחֻקַּי֙ וְלֹ֣א שְׁמַרְתֶּ֔ם שׁ֤וּבוּ אֵלַי֙ וְאָשׁ֣וּבָה אֲלֵיכֶ֔ם אָמַ֖ר יְהֹוָ֣ה צְבָא֑וֹת וַאֲמַרְתֶּ֖ם בַּמֶּ֥ה נָשֽׁוּב׃

From the very days of your ancestors you have turned away from My laws and have not observed them. Turn back to Me, and I will turn back to you—said GOD of Hosts. But you ask, “How shall we turn back?”

https://www.sefaria.org/Malachi.3.7

וְאָמַרְתָּ֣ אֲלֵהֶ֗ם כֹּ֤ה אָמַר֙ יְהֹוָ֣ה צְבָא֔וֹת שׁ֣וּבוּ אֵלַ֔י נְאֻ֖ם יְהֹוָ֣ה צְבָא֑וֹת וְאָשׁ֣וּב אֲלֵיכֶ֔ם אָמַ֖ר יְהֹוָ֥ה צְבָאֽוֹת׃

Say to them further:Thus said GOD of Hosts: Turn back to me—says GOD of Hosts—and I will turn back to you—said GOD of Hosts.

https://www.sefaria.org/Zechariah.1.3

Midrash Eicha Raba 

“Return us to You, Lord, and we will return; renew our days as of old” (Lamentations 5:21).“Return us to You, Lord, and we will return.” The congregation of Israel said before the Holy One blessed be He: ‘Master of the universe, it is incumbent upon You to return us.’ He said to them: ‘It is incumbent upon you, as it is stated: “Return to Me and I will return to you, said the Lord” (Malachi 3:7).’ Eikhah_Rabbah.5.21

. וְכַאֲשֶׁר יָשׁוּב הַמְקַצֵּר בָּהֶם בְּלִבּוֹ וּבִלְשׁוֹנוֹ וְיִשְׁתַּדֵּל לַעֲשׂוֹת אוֹתָם וְלֹא יִשְׁנֶה קִצּוּרוֹ בָּהֶם יִמְחֹל לוֹ הַבּוֹרֵא וְיִשְׁתַּוֶּה עִם הַצַּדִּיק אֲשֶׁר לֹא קִצֵּר בָּהֶם, וּבִכְמוֹ זֶה נֶאֱמַר הַשָּׁב מִן הַחֵטְא כְּמִי שֶׁלֹּא חָטָא וְאָמְרוּ רז״ל עָבַר עַל מִצְוַת עֲשֵׂה שֶׁאֵין בָּהּ כָּרֵת וְעָשָׂה תְּשׁוּבָה אֵינוֹ זָז מִמְּקוֹמוֹ עַד שֶׁמּוֹחֲלִין לוֹ שֶׁנֶּאֱמַר (מלאכי ג) שׁוּבוּ אֵלַי וְאָשׁוּבָה אֲלֵיכֶם.

The explanation of the first case: That the lacking was in a positive commandment which does not carry the punishment of Karet (spiritual excision), such as tzitzit, lulav, sukka, or the like. 

When the person repents on them in his heart and in speech, and exerts himself to fulfill them, and does not repeat his neglect to do them, the Creator will forgive him, and he will be equal to the tzadik who never sinned in them. On such a penitent, it is said: 'one who repents from a sin is as if he never sinned', and our sages said of them: (Yoma 86a) 'one who transgressed a positive commandment which does not incur Karet (spiritual excision) and repented - he is forgiven right away, as written: 'return to Me and I will return to you' (Malachi 3:7)'.

https://www.sefaria.org/Duties_of_the_Heart,_Seventh_Treatise_on_Repentance_8.2

Four motives in matter of repentance 

הָרִאשׁוֹן – הַשָּׁב מֵחֹזֶק הַכָּרָתוֹ אֶת אֱלֹהָיו. וְהוּא כְּמוֹ עֶבֶד הַבּוֹרֵחַ מֵאֲדֹנָיו, וּכְשֶׁחוֹשֵׁב בַּטּוֹב אֲשֶׁר גְּמָלוֹ, שֶׁמֵּרְצוֹנוֹ מְבַקֵּשׁ מְחִילָה מִמֶּנּוּ. וּבְכָמוֹהוּ אָמַר הַכָּתוּב (ירמיהו ד א): ״אִם תָּשׁוּב יִשְׂרָאֵל נְאֻם יְיָ אֵלַי תָּשׁוּב״ – קֹדֶם בּוֹא הָעֹנֶשׁ. וְאוֹמֵר (מלאכי ג ז): ״שׁוּבוּ אֵלַי וְאָשׁוּבָה אֲלֵיכֶם״.

The first is when a man repents because he has come to recognize his God. And he is like a servant who flees from his master, but when he thinks of the good which his master has done to him, he returns to him of his own free will to seek forgiveness from him, and of such as him the Scripture says, 'If thou wilt return, O Israel, saith the Lord, yea return unto Me' (Jer. 4:1). And further, 'Return unto Me, and I will return unto you' (Malachi 3:7).

https://www.sefaria.org/Orchot_Tzadikim.26.62

Natan Nachman 

Moshe Katz 

הֲשִׁיבֵ֨נוּ יְהֹוָ֤ה ׀ אֵלֶ֙יךָ֙ (ונשוב) [וְֽנָשׁ֔וּבָה] חַדֵּ֥שׁ יָמֵ֖ינוּ כְּקֶֽדֶם׃

Take us back, O LORD, to Yourself,And let us come back;Renew our days as of old!

https://www.sefaria.org/Lamentations.5.21

Tisha B'Av at night: The scroll of Lamentations. It is the emotions of the prophet Jeremiah's soul and his dirge that he took up about the destruction of the holy city of Jerusalem and the Temple. And it is especially about the sin that Jerusalem and his people sinned, which is what caused that awful destruction to happen. And while glancing with eyes full of tears, he instructs and shows the path that leads to repentance and towards the revival of Israel, to renew our days as of old (alluding to Lamentations 5:21).

https://www.sefaria.org/Horeb.666.33

Kedushat Levi 

“Take us back, Lord, to You, and let us return, renew our days of old” (Eicha 5:21). To be specific, what does “[days] of old” mean? And one could explain this by way of the midrash: “And now, O Israel, what does the ETERNAL your God demand of you? (Deut 10:12) and it says in the Midrash: “Now is nothing other than Teshuvah” (Bereshit Rabbah 21:6) and it is explained thus, because each person from Israel is obligated to believe with full faith that at every moment we receive life from the Creator, Blessed Be, as it is explained, “Every [thing that] breathes praises — every single breath phrases Yah, ” (Bereshit Rabbah 11, Psalms 150:6) — that in every moment, the living wants to leave a person, the Holy One Blessed Be, sends to [that person] in ever moment, new life. Because of this, for Teshuvah to be effective for each person, for at the moment that [person] does Teshuvah, that person believes that they are created with life anew and with this God, may God be Blessed, with God’s enormous mercy, does not remind them of their earlier sins. But, if, God forbid, they do not believe this, God forbid, the Teshvuah is not effective. This is the explanation of the Midrash, “Now is nothing other than Teshuvah,” since that [person] believes that they are made with life anew, the Teshuvah is effective for them. This is the interpretation of the verse, “Take us back, Lord, to You, and let us return,” and how is “renew our days of old?” this is what it says in the Gemara (Sanhedrin 98) [Rabbi Yehoshua ben Levi encountered the Messiah and said] “When will the Master [Messiah] come?”...“Today, if you will listen to his voice” (Psalms 95:7). When you [act] according to this quality, each day [a person] will be made with life anew.

https://www.sefaria.org/Kedushat_Levi,_Deuteronomy,_Megillat_Eicha.16

Avudraham 

And bring us back in complete repentance before You: is based on the verse, 'Bring us back, Adonai, to You, and let us come back; renew our days as of old.' (Lam. 5:21) In other words, if we begin to repent, You will help us to perform complete repentance before You. This is similar to the approach of the sages: 'If one comes to purity, they assist him.'

לשונו. והיינו, כי ישראל שואלים שיפתח להם הקדוש ברוך הוא הרהורי תשובה, כענין הכרוז שמכריז שהם הרהורי תשובה הבאים, והקדוש ברוך הוא משיב כי צריך התשובה במעשה שיתחילו בה.

https://www.sefaria.org/Reshit_Chokhmah,_Gate_of_Repentance_1.22