You will have to send a support ticket (click 'Need more help? Contact us!')
Send a support ticket saying that you would like to change your e-mail to another one, be sure to say which. They will then respond in probably 1-2 business days either confirming that you own it by asking your latest 3 transaction ID's, or they will send an e-mail saying 'Are you sure you want to change the e-mail to: email'.
Respond to the e-mail, and hopefully this whole thing will be done within a week.
You will need proof of transactions (if you bought realm gold) or some other proof.