How do i add or remove a dependent from my 2020 return if the irs has accepted my return already

Do not make changes to your filed return.  The amendment form will not be available until Feb. 28th (subject to change).   You will have to wait until you can amend your 2020 return.  The IRS suggests you wait until your original return has been fully processed (refund received or payment withdrawal) before you amend.