Click on userID to edit user. If you change the username, the system will check that the username is not already taken. You cannot edit the password, only change it. Be sure to verify that you entered the password correctly.
You can also choose to delete the user(s) from the user database. Simply check the checkbox next to the user(s) you want to delete, and then click on the "Delete Users" button at the bottom of the list. Be sure to check the checkbox following the Delete button.