Right above your post, you should see a link called "My Stuff" with an arrow pointing down. Click the arrow, then tap "Profile". To change the email address associated with your forum account, simply enter (and re-enter) your email address in the fields labeled "Email Address:" and "Verify Email Address:"

Click Submit (very bottom of the page) and you will need to visit your "new" email address for a verification email. Once those instructions are followed, your new email account will now be associated with your account.