This hoverboard doesn't have any unlockable upgrades.
Hammer is a limited hoverboard that was released with Subway Surfers World Tour: Copenhagen. Like any other hoverboard, it protects you from crashing for 30 seconds. When crashed, it will explode and recharge for a few seconds before it can be used again. It has the appearance of a golden hammer. This board costs 50,000, like most of others limited hoverboards. When landing, thunders are shown as visual effects.