What I would do is this. Insert double pointed needles in a destination row (see video on this page
if unsure how) about 1/2" down from the cast on edge, making sure that you get ALL of the stitches of the same round onto your needle. Take a scissors and cut the cast on edge off. Unravel those few rows until you reach the stitches that are on your needle. Re-join your yarn and work back the other way until your cuff is the right length again, then cast off VERY loosely with a needle size at least 2 up from what you used for the knitting.