At one point facebook had "master admins" which could not be deleted as they were the creator of a page. This has since been removed and now any admin of the page can remove another (keep this in mind when you decide who to give the keys to your page too).
To change admins simply go to the "Edit Page" --> "Admins" option on the brand page. If you are not an admin yourself you will need to be added as one first.
Facebook handles all logins via personal accounts which are linked to brand pages. There are no direct tools to login as a brand although many brands use ghost (empty / null) accounts to control access to their pages.