how to amend my 2021 tax return on turbo tax
From what to what? You can't amend to change from Joint to MFS after the due date of the return. See note at top of this article
https://ttlc.intuit.com/turbotax-support/en-us/help-article/small-business-processes/switch-filing-jointly-filing-separately/L3UGzwHa1_US_en_US