You have to buy and do each year separate. So you need a 2017 program and a 2018 program.
Did you start both years using Turbo Tax Online? Did you file 2017 or need to finish it? You do not amend if you did not file.
How to finish a prior year online return
https://ttlc.intuit.com/questions/2559403-how-do-i-finish-a-prior-year-return-that-i-started-in-turb... Do you want to switch to the Desktop program? If you started 2018 online we can tell you how to move it to the Desktop program.
You can buy 2018 here,
https://turbotax.intuit.com/personal-taxes/cd-download/ You can buy 2017 here,
https://turbotax.intuit.com/personal-taxes/past-years-products/
