https://accountants.intuit.com/support/en-us/help-article/amend-tax-return/amend-individual-return-1...

In particular

To amend a previously amended return:

1. Open the amended tax return.
2. Go to the Input Return ⮕ Miscellaneous Forms ⮕ Amended Return (1040-X)/Superseded ⮕ General Information and Federal (1040X).
3. Remove all information pertaining to the amended return on this screen.
4. Review the 1040 to ensure the information matches the prior amended return.
5. Go to the Input Return ⮕ Miscellaneous Forms ⮕ Amended Return (1040-X)/Superseded ⮕ General Information and Federal (1040X).
6. Mark the checkbox labeled Amending federal return.
7. A window will open asking if you wish to automatically fill out the As Originally Reported amounts.
   • Select the box to Make a copy of this return if you haven't already done so.
   • Select Yes, transfer data.
8. Enter a 1 in 1=amending state return, if applicable.
9. Enter the Year to amend (MANDATORY).
10. Select the Explanation of Changes tab and enter an explanation to print with the return.
11. Select the Gen. Info. & 1040X tab.
12. Fill any applicable fields in the Miscellaneous Information section.
13. Scroll down to the Federal Income Tax Return section.
14. Fill out the As Originally Reported column using the amounts from the original 1040.
15. Use the regular input screens to make any changes to the return.
    • For example, to report additional wages, go to the Wages, Salaries, Tips (W-2) screen and add a W-2.
    • Don't use the Correct Amount [Override] column unless you're amending a prior-year return in the current year program, or it's absolutely necessary. These overrides will stop any automatic calculation and require you to manually calculate the new tax amount.
16. Review the 1040X for accuracy using the Check Return tab.