Bitcointalk accounts are not made for selling so this isn't going to be feature that everyone needed but you can change your email of your bitcointalk account if you know the password just by Profile= > Account related settings=> enter new email and enter current password and its done.
Yes its not but it can be used to do selling activity such as digital goods and stuff which is allowed in here as long as they are following the rules. Of course with the exception of the bitcointalk account which is highly prohibited activity here.
As far as I know OP email can be changed maybe you arent noticed it, just head to profile section and edit it.