What do you mean?  Did you already file and now need to amend it?

How to amend an Online return

https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-turbotax-online-return/00/27577

I just put 2018's info instead of 2019.

Has your return been accepted by the IRS?

• NO. If you have not filed the return, simply change the information. Revisit your forms and change the numbers.
• YES. If you have already filed, you will need to amend your return.
  • First, your original return must be financially settled, all paperwork complete before you do an amended return.
  • Where's my refund - for original return if finances are not settled
  • file an amended return - how to amend
  • track an amended return - track the amend

It only says I can only amend by mail.

Amended returns (1040X) must be printed and mailed.