May I help you? and How may I help? is actually the same meaning ka.
But May I help you? is more of the offer that I give to you with out you asking me.
How may I help? is more likely when you ask me for help and then HOW can I help you. But also can be ues in the case of someone walk into the shop or the company without asking for any help. Then the assistance or the reception ask that person that could they be any help to you(that person).