goto options->security->saved password, look for the one you want to remove, select it, then remove it.

Note that you can use these steps to remove saved form data or name/password data from a drop-down list:

1. click the (empty) input field on the web page to open the drop-down list
2. highlight an entry in the drop-down list with the mouse or cursor Down key
   do not click the mouse or press the Enter key
3. press the Delete key (on Mac use Shift+Delete) to delete the highlighted entry