Loging out (deleting authorisation) to log in with new mail address and password

I just downloaded digital editions to be able to read some books. It seems it has logged automatically in with a very old mailaddress of mine (which I don't have anymore). The only way to log out is tot type in the password of the old mail address, but I don't have that any more and I can't access the mailaddress anymore either. How can I start all over with a new mail address and password?