If you want to add a new Administrative user, add a username for them in the first text box and hit enter (or click on the "Add User" button). Note that you can't add a new user with a username that already exists.
To change a user's password, select their name in the first dropdown list. You may or may not have to click on the "Edit User" button, depending on whether or not you have Javascript enabled.
To delete a user, select their name in the second dropdown list. You may or may not have to click on the "Delete User" button. Note: you are not allowed to delete the "admin" user. This is just to make sure you never lock yourself out of your admin section by deleting all the users in it.