How do I make a change to a printed but not mailed return form

You can go back in the program by selecting add a state (you won't be adding a state but this will get you back into the program). Please follow the steps below:

1. Login to your TurboTax account.
2. In the Welcome Home box, click on the My TurboTax tab at the top and then My Tax Timeline
3. In the SOME THINGS YOU CAN DO section, click Add a state. (Important do not select continue with state return as it will send you back in a loop. Please select add a state, as this will bring you back into the program, even though you are not really adding a state)
4. Now you are back into the program and can make the changes to your Federal return. Once you make the required changes please make sure to review your return carefully before printing and mailing.

Thank you so much for the quick and helpful answer.

You are very welcome!

This doesn't work for me. Just brings me back to a screen that says "It looks like your federal return can't be amended right now"after click "add a state". I've completed & printed but net sent my return and want to make a change

Please Note: The Welcome Home is now the first screen that you see after signing back into your account online. "Add a State" is one of the things that you might still need to do after choosing to print and mail a "Federal and/or State" return or after electronically filing just the "Federal" (some "States" may not accept a return until the "Federal" is filed first (by mail or electronically)).
Please Note: If you electronically transmitted or mailed a "Federal and/or State" return already, you want to refrain from trying to electronically transmit or mail a return twice without going through the steps to "Amend" your return. The online platform products will NOT allow you to electronically transmit more than one (1) "Federal" and one (1) "same State" return.
Please Note: If you mail a second return to the IRS without following "Amendment" guidelines, before or after the IRS accepted another return that you sent for the same tax year, by mail or electronically, for the same tax payer, doing so will have a negative impact on the validity of both returns. Without following proper procedures to "Amend" a return (see below), both returns are subject to getting rejected by the IRS, and the timeliness of the return that you might deem most accurate to file, could be negatively impacted. 

See https://turbotax.intuit.com/tax-tips/amend-return/how-to-file-an-amended-return-with-the-irs/L6kO691...

The purpose of this comment is to add more detail.