I need to adjust my 2015 return.

In January 2016, I made an $11,000 contribution to a traditional IRA and then converted that amount in its entirety to a roth IRA (what I understand is a backdoor roth).  I intended to have half the amount apply to 2015 and half to 2016 so that I would stay below the limits.  I think I need to adjust my 2015 return to reflect this, but I am not certain.