How do we go back to 2020 and file a form 8915E for covid disaster IRA distribution?

Did you report the 1099R distribution at all on your 2020 return?  Is it on 1040 line 4a and how much on 4b?   You might have reported 100% in 2020, in that case you are done and don't need 8915 to split the income over 3 years.  

8915-E is only for tax filers who were affected by Corona Virus in tax year 2020.

If you were affected and your age is below 59 1/2, you paid a penalty for early withdrawal that is waived.

You'd have to amend.

@tcnmiller1988