You cannot do it. And in the process of signing up they mention it, there is no way to change your password.
They generate random password for your own protection, because some stupid people make passwords like "qwerty123" or put their names as a password.
This was annoying until I got roboform, it's the most useful tool there is. I don't even know my clickbank password lol, but roboform does.
You can't change it unfortunately, I would personally prefer a more secure password. I typically generate a random 10-12 character password for such things