If you had carried out a simple Internet search you would have found the following.
Open the Settings app on your iPhone.
Scroll down and tap Apps, then select Mail.
Tap Accounts (or Mail Accounts) and select your BT email account.
Tap the Account Name to enter the settings.
Under Incoming Mail Server, delete the old password dots and type in your new password.
Scroll down to Outgoing Mail Server and tap on SMTP.
Tap the active server.
Tap the Password field, enter your new password, and tap Done
