@Muse9876
You get the 1040-X as a fillable PDF at the IRS website and put "2020" in the tax year spot above your name and address.
Form 1040-X is used for many purposes, which you state in Part III.
Since you already took your $3,000 max deduction for losses and that is the correct number,
then you are not changing any original amounts on Form 1040, and you don't have to fill out lines 1-23.
Leave that all blank.
Attach the corrected Schedule D (pages 1-2) to your Form 1040-X with an explanation in Part III.
Mail to the address specified in the "Instructions for Form 1040-X", also online.
