You can change your keys by going to https://steemworld.org/@throwbackthurs and there scrolling down to "Change Password", where you can change your password.
You can probably do this in at least a dozen other ways, but steemworld is probably the one I'd use.
But as @enforcer48 already said, make sure that they don't have your posting authority either, since then your new password won't stop them from posting in your name.