User name

I am getting up the old usernames when I book in to say a yahoo site, when I go to type my username all the other user names I have used in the past come up in a drop down menu, hope this makes sense , so for instance I want to set up a account with a company I put in a user name and password to get started, but at a later stage i want to change my user name which I can do with no problem, however when i go to enter my new user name I keep getting a drop down menu with the previous user name...... How do I delete them?

Thank You