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?
Best Answer
You cannot upcast spells from the Staff of Power outside of those it "forces" you to upcast (such as 5th-level fireball)
The description of the Staff of Power (DMG, p. 202) explicitly describes what level its spells are cast at, and gives a single number for how many charges it costs:
I won't include the rest of the spells, cone of cold and fireball serve as sufficient examples; cone of cold would be cast at its base level1, and fireball would be upcast at the level it specifies (5th-level).
No further text seems to imply that the level that these spells are cast at can be in any way modified by using a different number of charges (not only upcasting, say, cone of cold or fireball to 6th-level or higher, but also there's no way to cast fireball at 3rd- or 4th-level by using fewer charges, for example).
As a counterexample, consider the description of wands; let's take the Wand of Fireballs as our example (DMG, p. 210):
Since the Staff of Power does not describe that you can upcast using additional charges, whereas the Wand of Fireball does, then by omission, we must conclude that the Staff of Power cannot do this.
1 We know that spells cast from magic items are cast at their base level unless specified otherwise because of the generic information on spells cast from magic items from DMG, p. 141 (under the heading "Spells"):