To change the email you use to sign in, you will need to do so from the Preferences page.
NOTE: Only the user themselves can change their sign in email. Even if you are an administrator, you will be unable to change another user's sign in email.
Click your preferences icon in the upper-right and then click User Profile.
Change the Sign in Email to one you wish to use. You cannot use an email that is already associated with a Method:CRM Account. Doing so will result in an error.
Click the Save all changes button.
Sign out of your Method:CRM account.
You can now sign in with your new sign in email!