I have been having this problem myself, and just stumbled on the answer.
You have to (while logged in) first, click on the "Registry" (at the top of the page between "Community" and "Downloads")
It will bring you to a new page. In the upper right hand corner of the window, should be your profile name. Click on the profile icon in the upper right and a menu will pop up with "edit profile", "help", and "sign out" on it. Click on "edit profile"
A new page loads where you can change your email address. Enter your email address in the box, and click "Save Profile"
You will have to confirm your new email address.
Why you can't change the address from the "Contact Details" page I don't know. If you can't change it from that page it should just say "Change it from the Registry tab" or something. Hope it helps!