went to a vz store and asked, then actually swapped it for a 6S.
basically you can either apply it towards the "financed" amount, so your monthly drops by $12.50 (300/24), or use it towards the monthly payments, or another purchase. i actually wound up taking advantage of the $300 off any iPad offer, so wound up getting a 64GB iPad air 2 for $130 + tax ($730-300 for promo - 300 for phone exchange)