You have to change the password on your BT mail account if you forget your current password or you frequently change your password as a security measure. Since BT's mail services are powered by Yahoo!, regular uses of the Yahoo! Mail system will be familiar with the process for changing a password. You can change your password as often as you like.
Visit the BT.com password reset page. It is linked below in the References section, under the heading "BT Yahoo!: Reset Your Password."
Enter your BT e-mail address in the text field, and select the domain on which your BT e-mail account is located: btinternet.com, btopenworld.com or talk21.com. Click "Next" when complete.
Answer the two security questions connected to your BT e-mail account and click "Next."
Select the new password and enter it twice on the "Reset password" page to confirm it. Click "Next."
Review and confirm the updates to your account on the subsequent "Confirm details" page. Upon reviewing the changes to your account, click "Next" to confirm them.
Update your newly-changed password in your e-mail client and on your smartphone. Failure to do so will disrupt the synchronisation between your BT e-mail account and other devices.