If you are finding errors with your data entry within the program then you can simply go back and click back on that section to revisit the questions.
If you had marked your return for "file by mail" and after printing you found errors (and you haven't mailed anything off) that you'd like to correct then you should follow the instructions below to change your return status from "file by mail" to e-file. This will "unlock" the program for you to make changes and then you can change your filing status back to "file by mail" when you are done.
https://ttlc.intuit.com/replies/3985787
