How do you use your xp reserve on urban rivals?

Under the card of the character you wish to level up in the "My deck" or "my collection" screen you will have the option to "sell | (add __ xp / Up! __ xp) and the option on the right will add XP. if it says "Add *blank* Xp" then you dont have enough XP to level up your card. if it says "UP! *blank* XP" then you have enough in your reserve to increase the level of your character.