First log into your account with your existing email address, then you can invite your other email into the account.

Go to the main account settings - my team.

Click to invite another teammate

Send your new email an invite and accept it and you're in.

You can now remove your other login from the account.

If you're the main billing contact then it won't let you remove the user. Please go to the Account Settings page to update this first and then return to the team page to remove your email.