You will have to amend the 2020 tax return for the W-2C.
However, if the only thing that changed on the W-2 was the Social Security taxes there is no need to amend the 2020 tax return.
See this TurboTax support FAQ for amending a 2020 tax return - https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-return/00/27439
