How do I file an amendment

You cannot file an amended tax return until the IRS has accepted your federal tax return.

The IRS will not be accepting any 2020 federal tax returns until late January 2021.

You will then have to wait for the IRS to process the federal tax return and issue the federal tax refund.  After you receive the tax refund you can file an amended tax return.  The amended return has to be printed, signed, dated and mailed by you to the IRS.

The IRS will take 4 months or longer to process the amended return.

See this TurboTax support FAQ for amending a 2020 tax return when using the online editions - https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-turbotax-online-return/01/27577