When you go back through the File section and reach the following page, select Edit.
On the next page, select the second option "I can't get my 2018 tax return. I'll print and mail or use the self-select PIN I created last year." Then select Continue.
On the following page, choose the second option, "Use the 5-digit self-select PIN I created last year..." Then select Continue.
You will then be able to enter your PIN and re-file your return.