I received a w2-c for tax year 2020. Can I add that onto this year's taxes or do I have to file something separate?

Sorry you will have to amend 2020.  What changed on it?  You might not need to amend.

https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-return/00/27439 

@VolvoGirl It was that period at the end of 2020 where they didn’t take social security taxes out for a few months. At the beginning of 2021, they took extra out to make up for that.

You don't have to amend for that.  Unless you had more than 1 employer and all together they took out too much Social Security .  Then you can get the excess back.

For 2020 the max for Social Security is $8,537.40 on $137,700 of wages.