Since the Staff of Power says:
While holding this staff, you can use an action to expend 1 or more of its charges to cast one of the following spells from it
Can you expend additional changes to cast a spell at higher level?
Example: With the Staff of Power you can cast Magic Missile of level 1 (1 charge), can I use more charge and cast it at higher level? So maybe use 2 additional charge and cast a level 3 Magic Missile?