In the Form 1040-X PDF on the IRS web site, read the "New Information for Form 1040-X Filers" at the top of the cover sheet on the page before the actual form. It says the following.
"If the taxpayer is amending their 2020 calendar year return, they should manually enter "2020" in the entry box next to "calendar year" on the line below the calendar year checkboxes."
