How do you teach someone with dementia to use electric wheelchair?

Depends: If the dementia is mild, one may be able to teach someone with dementia the same way one teaches anyone. But more likely the person with dementia will have trouble learning.

More reps may work.: The most important way to learn any new skill is by repetition. If the dementia is of the type or severity that does not significantly impair new learning, extra repetitions may do the trick. But if it is of the type that primarily or significantly affects new learning, then all bets are off and it might be impossible to teach any new skill.