How do i file a amended 2015 return
If you used TurboTax to complete and file the original 2015 tax return then see this TurboTax support FAQ for amending a 2015 tax return (online or desktop) - https://ttlc.intuit.com/questions/3410543-how-do-i-amend-change-or-correct-a-2015-return-in-turbotax
If you did not use TurboTax to complete the 2015 tax return then you can get free forms, schedules and instructions from this IRS website - https://apps.irs.gov/app/picklist/list/priorFormPublication.html
IRS Form 1040X Amended Tax Return - https://www.irs.gov/pub/irs-prior/f1040x--2016.pdf
IRS Form 1040X Amended Tax Return Instructions - https://www.irs.gov/pub/irs-prior/i1040x--2016.pdf