THis depends. Generally, if you made payments to both lenders at any time during the tax year, you will receive a 1098 - Mortgage Interest Statement from each lender. It is important that you enter each 1098 in order. Enter the one for the old mortgage first. Pay close attention to detail and read the small print on each screen - every single word. If I recall correctly, there's a box that reads something like "This loan was paid off in 2016". If so, select it.
Then enter the 1098 from the 2nd (new) lender and make sure you check the box to indicate that this is a refi. Then go from there.