If you have not yet mailed the return, then yes, you can make changes to the return.
In order to get back into your return to make changes, you will need to indicate that you are trying to Add a State. This will open up the return again so that you can edit it.
The step by step instructions are in the following TurboTax article, just stop after step 4 and make your changes:
How do I change from mail to e-file in TurboTax Online?