How do I amend my 2020 tax return to show that I have received my stimulus money?

When you are amending the 2020 tax return -

Enter stimulus in the Search box located in the upper right of the program screen.  Click on Jump to stimulus

Do not amend for that.  The IRS will catch it and adjust your return.  They know they sent it to you.  

If you entered an incorrect answer regarding the amount of stimulus you received then you have an amount on line 30 of your Form 1040.  The IRS is checking every return with anything on line 30.   This will delay your refund, but they will catch the mistake and correct it.

The stimulus credit error doesn't have to be corrected at all since it doesn't affect the current or future returns ... don't bother.