If you used TurboTax (TT) for 2020, that should happen automatically.
To verify, In TurboTax (TT), enter at:
Federal Taxes Tab (Personal for H&B version)
Deductions & Credits
-Scroll down to:
--Estimates & other taxes paid
-Other Income taxes
On the next screen, scroll down to:
-2020 Refund applied to 2021 Taxes
-2020 Refund applied to 2021 Federal Taxes
On the next page, the box should be prepopulated with last year's refund. If not, change it to the correct amount.
