This passage of Jonah's prophecy depicts the repentance of the Assyrian Ninevittes following Jonah's brief message of judgment. A review of Jonah's prophecy of judgment from verse four will set the stage for a greater understanding of the mass repentance described in verses six to ten.
"Jonah began to go into the city, going a day's journey. And he called out, Yet forty days and Nineveh shall be overthrown!" (3:4)
'will be overthrown' (ESV/KJV/NASB) / 'will be overturned' (NIV)

nehpaket - the basic idea is 'to turn' ... but the word is used elsewhere in the OT:
destruction (Gen 19:21)
transformation (Ps. 66:6)

So literally, God declares through Jonah that within 40 days of this declaration in the open square, Nineveh would be changed.
Change was coming on way or another.
Jonah 3:2 shows us that Nineveh was a great city. The Hebrew connotation here could be translated, "Nineveh was a great city to God"