If you are using the CD/downloaded TurboTax Home and Business version, you should be able to open your tax file and edit the section for the additional deduction. Then go back through the process to print a new extension form.
Since the extension was not electronically filed from this version of TurboTax, you are not affecting anything you've already done unless you already mailed the extension.
If you have mailed it and included a payment, do not be concerned. If you were entering another deduction, then the amount you may have already paid should cover your taxes. Everything will be reconciled when you complete the return and file it.
Thank you! I went in and searched "extension." I found the extension form and deleted it. Now I can edit my federal taxes.
I also had printed a Pennsylvania extension form. I could not figure out how to delete the Pennsylvania extension form. So, I went to forms, opened the extension form, and made my payment 0. Now I can review and edit my Pennsylvania return.