If you used Turbotax to file your original 2013 tax return, you can use Turbotax to amend that tax return, and it is free. You will need to log into the account that you used back in 2014 when you filed your 2013 tax return.  I have included a LINK below that will help you get started with your amend.

https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed