If you've already mailed the return or have e-filed already, then you need to file an amended return. Turbotax has an option for that.
If you have NOT mailed it in nor e-filed, then go back to the tax return on Turbotax. Open up the return. Click on the tabs for FEDERAL TAXES near the top (just under the red checkmark). It will bring you back to the beginning. You can add any new forms you need now. Usually, when I realize I need to add in a new form, I go through everything one step at a time to make sure it is all correct and updated. It should be fine. Go through the State Return as well so that any changes are sure to trickle down to that return as well. Then print out once more. Should be good to go.
