How do I change the year on my return?

If you have not filed, you can simply change it. 

If you have already filed, here are the steps to amend a return:

https://ttlc.intuit.com/replies/7360890

Keep in mind that TurboTax Online can only be used to prepare tax returns for the Current Tax year (right now only 2018). If you need to file a 2017 tax return, you'll need to use TurboTax Desktop. https://turbotax.intuit.com/personal-taxes/past-years-products/