How do I file for recovery rebate if already filed taxes?

Did you not file for it because you had already received the first two stimulus payments?  Are you a dependent?

You file an amendment and add the recovery rebate credit.  The credit is for the first and second stimulus payments.   If you qualify for them and did not receive them or did not receive the full amount you qualified for, you can claim the recovery rebate.   The link explains how to claim the credit.   But you will need to amend your tax return if you have already filed (and accepted) and did not claim it on the original return.