1) Through the ReadCube desktop
Choose "Account" and update your email. Don't forget to click "Update my Account"
2) You can also change your email by updating your email through ReadCube.com. Go to ReadCube.com/Account (or just click your name/sign-in in the top right corner).
Click "Edit" and update your email. You will need to verify your new email - so don't forget to check your email to ensure the change is confirmed.