To go back into the return, you can use the Amend option (without really amending). Here's how:
From Tax Home, select Amend and then use menu on the left side to go to the Federal tab, then go to Deductions & Credits, and scroll down to Your Home to access the mortgage insurance deduction section of the program.
If you accidentally start to amend, here's how to "un"amend:
https://ttlc.intuit.com/replies/4170678
