If the corrected amount in box 4 of the Form W-2c for 2020 causes the total amount of employee Social Security tax (or equivalent portion of the Tier 1 RRTA tax) withheld by all of your employers to exceed the maximum amount ($8,537.40) of tax that you owe, or increases an already existing excess amount of employee Social Security tax (or Tier 1 RRTA tax withheld), then you should file Form 1040-X, Amended U.S. However, if you had two or more employers in 2020 and your Form W-2c for 2020 shows a correction to box 4 (or to box 14 for employees who pay RRTA tax) to account for employee Social Security (or Tier 1 RRTA tax) that was deferred in 2020 and withheld in 2021, you should use the amount of Social Security tax (or Tier 1 RRTA tax) withheld reported on the Form W-2c to determine whether you had excess Social Security tax (or Tier 1 RRTA tax) on wages (or compensation) paid in 2020. If you had only one employer during 2020 and your Form W-2c, Corrected Wages and Tax Statement, for 2020 only shows a correction to box 4 (or to box 14 for employees who pay RRTA tax) to account for employee Social Security (or Tier 1 RRTA tax) that was deferred in 2020 and withheld in 2021 pursuant to Notice 2020-65, no further steps are required. I'm not able to link it directly in this message for some reason ( [product key r.), but go to irs.gov/forms-pubs, select 'Post Release Changes to Forms' and scroll down to 'Form W-2 Reporting of Employee Social Security Tax Deferred under Notice 2020 65' to read the full context.

Here is some guidance from irs.gov I found while researching this same message in Turbo Tax.

Download w2c 6