There is no charge to file a amendment to your 2016 return but if you are needing to amend a prior year, you will have to purchase the desktop version of the software to amend that return. And you can find the desktop versions here https://turbotax.intuit.com/personal-taxes/past-years-products/

See below for details on how to amend a return you already filed.

https://ttlc.intuit.com/replies/3288565