After you file

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...>
Don't worry -- even tho the title says 2015 -- it tells you how to amend your 2018 return.