How did/do native Americans shave their heads/faces?
First, some Native Americans did not have heavy beards. They had very sharp knives and water, so I am sure they manages their beards and their heads that way. If they knew how to scalp in seconds, they could shave in a few minutes and be more careful I am sure. Before that, very sharp cutting tools like obsidian perhaps which can be sharpened like a blade.
First, white men introduced and practiced scalping (look up your facts!). Second, their are several ways for them to shave the limited amount of facial hair they produced anyway including hand-held implements, salves and poultices. Remember, they were able to cope and survive with a number of everyday tasks a long time before the barbaric Europeans came over and slaughtered them and shoved them on arid, unfertile reservations.