How do I tell TurboTax to apply my California refund for 2020 to my 2021 taxes? I can find how to do that for Federal, but not for California.

To apply this year’s state refund to next year’s return in TurboTax Online:

1. Sign into your account, and click continue from where you left off.
2. Click State in the left panel, then continue through your state return.
3. When you’re going through your state return, you’ll reach the A few things before we wrap you your state taxes screen.
4. Scroll down to Other forms and click Start or Revisit next to Estimated Taxes for Next Year.
5. Answer Yes that you want to start the state estimated taxes section now, and make entries on the next few screens as appropriate.
6. Go through the process to generate estimated payment vouchers to apply the refund to the estimates.

Both the IRS and states allow for the carryforward of refunds to subsequent year returns.  Please note that California has extended the state tax filing and payment deadline for individuals to May 17th, 2021.