To delete an individual stored password in Internet Explorer
please:

• Start typing your username eg “m”
• When the full
  username appears  as an option below eg “martin”  move the mouse over
  that so it is highlighted.
• Hit the delete key and you should get
  a pop up box saying “Windows has a password stored for this username.
  Do you want to delete the username and password?”
• Click yes.
• You
  should then be able to save the correct username and password by
  entering them again and clicking the “Remember password” option. (Please
  note that you need to have the
  prompting feature enabled in the browser)

In Safari just enter the correct password and you will
get a pop-up box saying “This password is different. Would you like to
replace the saved password with this new one?” Click yes.

In Firefox the system is similar to Safari
or you can search for and delete individual passwords at
preferences/security/saved passwords