From Your Account, go to Manage Account Users, and under "Add an Account User," enter the new e-mail address. This will generate an invitation e-mail to the new e-mail address. The recipient of the e-mail will need to follow the steps to activate the e-mail address, generating a confirmation code. The current primary should then confirm the new user in the "Review Invitations" section of the Manage Account Users page. As a final step, the current primary should switch the primary e-mail address under "Manage Account Users."