IRS sent us a letter changing the amount of our 2020 refund that we carried over to 2021. TurboTax picked up the original amount and I can't figure out how to change it.

You don't need to change it in TurboTax.  You would need to amend your 2020 tax return to change it in TurboTax.   When you go to file, you will still use the original AGI from your return as you filed it.   TurboTax will autofill with the number from your original return which will match what you filed.    

You can change it here

Federal on left

Deductions and Credits at top

Then scroll way down to Estimates and Other Taxes Paid

Other Income Taxes - Click the Start or Revisit

Next page scroll down to 2020 Refund Applied to 2021

Click Start or Update by the tax you applied