You can switch to e-filing as long as you have not mailed your returns in. Please see the below FAQ for instructions:
How do I change from mail to e-file in TurboTax Online? or How do I print and mail a return in the TurboTax for Windows CD/Download software?
I suspect there is a way if you go back to the very beginning of the filing process.
Navigate back to the Federal reviews and error checks and step thru sequentially from there.
Be forewarned though...if you have a state to file too, if you choose Federal as File-by-Mail, you are forced to do the same for any state forms too.
Don't forget that file-by-mail means that YOU have to print/sign/address envelops and take to the Post Office yourself .... AND IRS forms go to the Feds...and state forms to the state with entirely different addresses...they don't go in the same submission.