You cannot register an email address that is already taken by another user. If you change your own email address while testing your exploit, make sure you use a different email address for the final exploit you deliver to the victim.
Lab: CSRF with broken Referer validation
This lab's email change functionality is vulnerable to CSRF. It attempts to detect and block cross domain requests, but the detection mechanism can be bypassed.
To solve the lab, use your exploit server to host an HTML page that uses a CSRF attack to change the viewer's email address.
You can log in to your own account using the following credentials: wiener:peter