The IRS will not calculate the Recovery Rebate Credit for you so you'll have to amend your income tax return to claim it. Wait for the IRS to process your return before you amend.
At this time, you're not able to amend a 2020 income tax return. As of today, it's estimated that the IRS will finalize the 1040X form on 3/18/2021. However check the forms availability to see any updates to that date.
Generally, Form 1040-X, Amended U.S. Individual Income Tax Return, must be filed within three years of the date the original return was filed, or within two years of the date the tax was paid, whichever is later.