The current online program can only be used to file a 2019 tax return. If you used the current online product to prepare and file a 2018 tax return, you have not filed it. All the information is on 2019 forms. To file a correct 2018 return, you must buy, download and install the 2018 program on your computer: https://turbotax.intuit.com/personal-taxes/past-years-products/
The 2018 return must be printed and mailed by you. It can't be efiled.
To fix your 2019 return that has 2018 information on it, you will have to prepare, print and mail an amended return form 1040X. Here is how to amend a return: https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed
