I'd probably just use the updateUser mutation? Does that not work for you?

No, you can't updates a system's user email or pw with that mutation.

By system's user, are we talking about a defined type that you have made? I'm unsure as to what you're asking about.

It's defined by graphcool itself. I also use their basic authentication.

Aaah, ok! I don't know anything about graphcool, sorry!