As long as you have already filed the original 2018 tax return....and have either received the original refund....or paid the original balance due...….You can follow the instructions provided in the help page below:
1) The link below provides tow selections....one for folks who used the ONLINE software, and another or those who used the Desktop software....be sure you select the correct sub-link.
2) Essentially, you just follow their directions, start the special Amend process, and add in all your new data. The AMEND process monitors what changes are made. IF you used the Online software, and your added changes require you to upgrade to a higher version, then you will have to pay for the upgrade [for Online users, Premier ((or SE)) is required for stock transactions. Desktop folks can use any version].
3) When done, you print out your amended tax returns (Fed and state)....and you have to take it to the Post Office yourself to mail it in....it cannot be e-filed. Federal forms go to one address...state forms go separately to your state (if you are in an income taxing state)
https://ttlc.intuit.com/community/amending/help/how-do-i-amend-a-return-in-turbotax/00/27579
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*
