How can I show that the IRS corrected my 2021 tax year refund after I efiled? Now I need to amend the return but the program assumes I received the original refund amount.

Assuming the change in your return doesn’t affect subsequent returns when TurboTax brings over data from a prior year you don’t have to change anything. Or if you understand what the IRS corrected on your return just edit the original return so that it reflects the change in refund. 

Difference of 2,800 sounds like the Recovery Rebate Credit on line 30.  How much is on line 30?  Since you have the Desktop program you can just change your original return before you amend it.  You probably put down the wrong amount you already got for the stimulus.  So enter the 2800 on the stimulus screen and check it reduces line 30 by 2800.  

To get to the stimulus questions in TurboTax at any time, you can search for stimulus or recovery rebate credit then hit ENTER and there should be a Jump to link

If you are trying to trigger it,
Click on Other Tax Situations at top
Click on Let's get started
Answer some questions then Continue
Then click on Let's keep going
Next screen is for the Third Stimulus, if you qualify

Then you can amend the return.  Then if you need to carry the changes over to 2022 you will need to start a new 2022 return and transfer from the amended return.  Go up to FILE-NEW.  

 I was assuming you needed to amend for something else.  If you just want to fix your return to match what the IRS changed then NO you don't need to amend for that.  You don't need to fix it.  If it was 2,800 for a Recovery Rebate Credit error that won't affect 2022.  

Or why do you need to amend?