I want to know how to register a new email and make it the main email account.

Hi Tiago, 

Welcome to Box Community. Happy to assist you!

The steps for adding or modifying your email address are as follows:

1. Login to box.com
2. Go to Account settings (click the profile pic or initials located on the upper right corner)
3. Account 
4. Add a Collaborator or Link Personal Email 
5. Link Personal Email > enter the new email (non box email, yahoo. gmail. outlook, etc.)
6. Confirm the email

7. Go back to account settings and make the secondary email as primary and remove the original email.

I trust that the above steps will do the trick.