I filed my 2020 return, receive my refund. Had to amend, now owe, what do I do with the refund check?

[DoninGA]

You keep the refund check you received from the original tax return.  You pay the taxes owed from the amended tax return.

[Mike9241]

and make sure the amended return reflects that refund check.

[Critter-3]

Look at the form 1040X …

Column A should have the figures from the original return, Column C the corrected figures and Column B the differences between the other 2 which needs an explanation on page 2 of the form 1040X.

If you completed it correctly you will see an entry on either line 16 OR 18:

On line 16 should be the amount you paid with your original return. 

On line 18 should be the amount of your original refund you received.

Then you will see your extra refund on line 22 OR  the new balance due you need to pay on line 20.