Sign in and open your return, if you haven't already done so.
From the menu on the left, select File (tap in the upper-left corner to expand the menu on mobile devices). This will put you on the [Name], just X more steps and we'll file your returns screen.
Go to Step 3 (if needed, revisit Steps 1 and/or 2 to activate Step 3).
Proceed, and make sure you've selected the option I want to e-file.
Continue through the screens, and soon you'll see the place to enter your 2018 AGI.
Keep going until you’ve re-transmitted your return.
