How do I file an amended return for 2015?

Well you cannot use Turbotax ... so get the forms and instructions from the IRS/state   OR  seek local professional assistance.  Either way it must be mailed in. 

If you are looking for an additional refund - forget it.   All years before 2017 are closed and beyond the 3 year statute of limitations for refunds.

if you owe the IRS/state can come after you forever with increasing penalties and interest otherwise forget it.

TT no longer offers 2015 software (for desktop - full PC or MAC). if you buy the cd on the web it will not be updated so you could end up with an incorrect return. best advice if you owe see a pro or do it manually.