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

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

And here is the link telling you how to do it.
<a rel="nofollow" target="_blank" href="https://ttlc.intuit.com/questions/3410543-how-do-i-amend-change-or-correct-a-2015-return-in-turbotax">https://ttlc.intuit.com/questions/3410543-how-do-i-amend-change-or-correct-a-2015-return-in-turbotax</a>
Don't worry -- even tho the title says 2015 -- it tells you how to amend your 2018 return.

https://ttlc.intuit.com/replies/3288565  

https://ttlc.intuit.com/replies/6497686