Same question here. It looks like the playing site online-go.com and this forum forums.online-go.com have two different ways of doing this. At the playing site, yes, there’s settings → account settings where you can change it.
But here on the forums, there are no account settings! I’ve found a page called “preferences” which shows my address as “user-8776@lots of digits.mailrouter.online-go.com” which is not a real email address (definitely not the address I signed up with, and if I try to send email to it, I get an “undeliverable” message in reply). Underneath that address is the text “Email can be updated from authentication provider. Never shown to the public.” I’d like to replace that email address with my real address, so that I can get email notifications when people reply to me.