No, once your return is filed and accepted you cannot make any adjustments until it is processed by the IRS. After that you can amend your return for items like adding missing income forms (W2). You mention that you received a W2 from another year. Be sure you amend the correct year. The form W2 will show the tax year that it applies to.
If you missed a W2 the IRS will normally catch that error and send you a letter with instructions on how to correct.
If after your 2020 return is processed (refund deposited) you need to amend you will choose the "Amend" option when signing in to your TurboTax account. Follow the prompts to make the changes and submit your amended return.
If you need to amend a prior year return you will need to follow the instructions here.
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