Go to the "Button Configuration" screen at the options menu. Then, go down to "Select" and "Taunt" should appear next to it. Press Right until it reads "S.COMBO1 LVL1". The number after "S.COMBO" corresponds to what super combo you want to use. The number after the "LVL" represents what level you want the move to be. For example, to have Ryu use his Shinkuu-hadouken at the highest level, scroll until it reads "S.COMBO1 LVL3". This can also be done from the pause menu.