Totally! All you need to do is log in to your account and navigate to your "Account Settings" > "Edit Settings". Click on the "Edit" button to the right of your current email address. You will be sent a verification email (to the email that is currently on your account) that you need to open within 30 minutes of receipt, otherwise it will expire. Open the email (which will be from support@teepublic.com) and click on the link that says "Contact/Login Email Address". This will take you back to your account settings page and you will be able to edit your email from there. Make sure to scroll to the bottom of the page and click "Save All Changes"!
Please make sure not to use an email address that is or has been associated with another account.