Since you are using the 2015 desktop edition to complete the Form 1040X, just click on Forms. In Forms mode open the Form 1040X if not already opened. The refund is on Line 22. On Line 23 enter the tax year and amount of the refund you want applied to that tax year.
Return to the interview by clicking on Step-by-Step in the upper right of the program screen.
When you print the amended return verify that what you entered is shown on the Form 1040X to be mailed.
