First if you already filed there is nothing you need to do now ... if you did not enter the correct amount of the third stimulus payment that you got in advance when asked then the IRS will correct the error if your return has an amount on line 30 of the form 1040.
And if you were married and filed a joint return both years then you should each have gotten one of those "reminder letters". https://blog.turbotax.intuit.com/tax-planning-2/irs-letters-6419-and-6475-for-the-advanced-child-tax...
