After you file

@Mike237  Have no idea who you spoke with from TurboTax but they are mistaken.  The IRS Form 1040-X for tax year 2020 is available in all the TurboTax platforms, online and desktop, effective 03/26/2021.

See this TurboTax support FAQ on amending a tax return originally completed and filed using the 2020 TurboTax editions - https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-return/01/27439

Here is a screenshot of a Form 1040-X for 2020 from the TurboTax online Free edition -