Maybe the following will help:

1. Sign into your TurboTax account and select Fix my return.
2. On the screen, Let's fix your returns and then refile screen, select Fix it now or Revisit
3. On the following screen, enter your 2020 AGI from your Form 1040, line 11. Be sure to enter the number exactly how it appears. Then select Continue e-filing.
   • If you're fixing IND-032-04 for your spouse, be sure to use their Form 1040 and enter the exact number from line 11. Don't split, round, or try to calculate anything.
4. On the next screen, confirm that the number is correct and select This is my 2020 AGI.
5. Follow the instructions to finish and refile your return.

If you don't have your 2020 AGI, you can select File by mail instead, to print your return as-is and mail it to the IRS.

Also, see if you can change it in forms mode:

How do I switch to Forms Mode in the TurboTax for Windows software?

https://ttlc.intuit.com/community/using-turbotax/help/how-do-i-switch-to-forms-mode-in-the-turbotax-...