If they're correct and you failed to include it on the 2015 return, no amendment is necessary. You would just pay the bill. The taxes on that money was due back in April 2016, so the taxes are late and penalties and interest will apply.
You could try and request a waiver of some of the penalties, if you had a good reason for not including it on the tax return.
♪♫•*¨*•.¸¸♥Lisa♥ ¸¸.•*¨*•♫♪
