You can increase your Cooldown stat by moving your peg to the next open slot along your cooldown track. To do so, you must spend a number of XP equal to your new Cooldown stat value, which is printed above the next slot. For example, to increase your Cooldown stat to 3, you must spend 3 XP. You must increase your Cooldown stat one slot at a time, but you may increase it multiple times during the same Reward Phase if you have enough XP.