We have fired the company who supported our 2811 routers and I would like to know how to change the Administrator password so we can prevent their access while we are looking for a replacement company
Make sure the new one is working fine before you delete the old one.
the command to delete the old one is:
no username admin
You can reduce your risk by creating another account before deleting the admin account.
username administrator secret MyPaSsWoRd
Then create another telnet session (leave the original one open) and authenticate with the username administrator. If successful you can delete admin and re-create or just use the administrator account.