Your computer, per se, doesn't store your password. It's the specific program that you use to access your email which stores it - and you've not told us what that program is!
There are two completely separate ways of accessing a BT Yahoo account. Firstly, you can go to the relevant web page and sign in there. To do so, you simply use your normal web browser (such as Internet Explorer, Edge, Firefox, Chrome, Safari, Opera or whatever). If that's how you access your mail, please tell us which web browser you're using.
Alternatively you can use a dedicated email client, such as Outlook, Outlook Express, Windows Live Mail, Windows 10 Mail or Thunderbird. If that's how you're accessing your mail, please tell us which email client you're using.
Once we know how you access your email account we'll then be able to tell you how to delete passwords from the relevant program.