Open the return, hit CTRL + F (to Find forms) and type  X    It should pop up the 1040X in the list.

E Filing is closed for 1040 returns, so paper filing will be your only option...if its a refund you may want to wait for Efiling to reopen in January, would probably save some wait time.

(assuming you Efiled the original return, if it was paper filed or someone else Efiled it, you can't Efile the 1040X and the last sentence doesnt apply.)