You didn't say if you are using Online TurboTax or desktop software (CD/download), so I'll assume for this answer Online TurboTax.
Did you originally choose to print and mail the return, but have not actually mailed it? And now you wish to go back in and efile?
If you are using Online TurboTax, and if TurboTax thinks you are finished but you haven't actually filed by any method, here's how to open the unfiled return back up to continue working on it or to change the filing method. At the Tax Home click on "Add a State". (That doesn't really add one; that just opens the return.) Once the return is open, go to the REVIEW tab and be sure all is still in order. Then go to the FILE tab to change the filing method, and then proceed through the remaining screens to submit the efile.