If you filed with TurboTax there will be no extra fee for the software to do the amendments. We do not have those years online any longer, so you will need to download the software to your computer to complete the amendment but here are links to directions for 2015 and 2014.
2015
https://turbotax.intuit.com/personal-taxes/cd-download/amend/ty15/
2014
https://turbotax.intuit.com/personal-taxes/cd-download/amend/ty14/