Click a power in the power list so that it is no longer green. If that messes up your power selection level (the power bar that is a different color is your power selection level), click on a different power bar that you want to change to.
Example: Kick is in the level 49 slot; unselect it by clicking on Kick in the power pool list, and now your power selection level is at 49, but look at all those empty spots lower than 49! Click on the empty power bar for level 32 to continue selecting powers from that level.