As @Skylane said recreate the 2020 return as was original done (all forms, and schedules including state if applicably. You then activate the 1040X, and make necessary changes. For 2020 you have until 5/17/2024 to get a refund. from notice 2023-21.   "An affected taxpayer who filed their 2020 return after May 17, 2021, may be refunded all income tax payments for 2020 if they file their claim for refund by Friday, May 17, 2024."