if you are the main account holder, login to MyBT and visit the email management page under extras. You can change the password for the email here if you know the current password.
If you don't know the current password and you want to reset it, use this page at bt.com/forgotpassword and you can reset it directly for the email in question. If you don't remember the security Q&A or have a telephone number listed for recovery, only way is to call them and have them validate you which is a bit longwinded for obvious reasons.
