It's quick and easy! Just go here and login with your username/email address and password. Remember, your account with the RT Store is the same account with the RoosterTeeth.com website.
Then click on your profile picture in the top right corner and go to Settings. Click Account, then Profile. Here you can update your username, email, password, gender, the color of your username in live chat, or connect your Facebook account. (See below)
Select "Email", type in the new email address you want to use, click "Save" and you're done!