If you have filed with TurboTax, it is a fairly simple process.
For 2017, sign in at TurboTax.com with the login you used when you filed your 2017 return, then follow 2017 online amendment.
For 2016, sign in at TurboTax.com with the login you used when you filed your 2017 return, then follow 2016 online amendment.
