Can anyone tell me how do you file a uc-1099g after you have already filed your taxes?

 

After you file

You will need to amend your return. Not to worry. You have 3 years from today's date to do it.

After you file

And here is the link telling you how to do it.
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Don't worry -- even tho the title says 2015 -- it tells you how to amend your 2018 return.