Jaka jest różnica między Jak się masz? i Jak się miewasz? ?Możesz podać przykładowe zdania.

"Masz" comes from the verb "mieć", while "miewasz" comes from the verb "miewać." Miewać differs from mieć in that it describes a habitual state, something that happens from time to time:

Mam zgagę = I have reflux (right now or generally speaking)

Miewam zgagę = I have reflux from time to time

With the reflexive voice, however, this distinction is less strong, and you could say that both "mieć się" and "miewać się" more or less mean the same thing. The only difference is a stylistic one. "Miewać się" sounds more polite and formal than "mieć się." I would probably not say "jak się miewasz" (no reason to be formal with someone whom you're adressing with "ty" anyway), but I could consider it with "pan" or "pani" (jak się pan(i) miewa.) Still, you'll probably hear "jak się pan(i) ma" much more often.