Do I have to amend my Michigan State Tax Return in order to fix my Detroit City Tax Return? I need to fill out form 5120 instead of form 5118 but I can't do it on my own.

[JohnW15]

The City of Detroit doesn’t appear to require the filing of a Michigan state income tax return when amending your city return. 

And, since you just want to file a part-year resident income tax return instead of the resident income tax return you originally filed, there shouldn’t be a need to amend your state return.  You’d want to amend your state and federal returns if you had additional income to report, but based on what you’re saying, you're just changing residency status.

Instructions can be found at Detroit’s 2017 City Individual Income Tax Forms page.