I assume you are trying to resolve the issue by logging onto BT website then mybt/mail top right bar.
Try this Google to 'yahoo.co.UK' when the page opens to yahoo home,scroll down looking left side for 'Mail' the log on panel will appear right side of screen go from the. The change password option is there if you need it.
To use this a non yahoo addy is preferable (gmail?)to do the change,which can be he original.