You can't use the online product to file a 2015 tax return. It is only for 2016. If you want to keep the account you already used to mistakenly prepare a 2015 tax return, you can clear and start over and do your 2016 return.

Here is a link to help you with preparing your 2015 tax return: https://ttlc.intuit.com/questions/2559403-how-do-i-finish-a-prior-year-return-that-i-started-in-turb...