It normally is helpful if you provide the model type because there is a different way to do this in the older gen 1 units and the somewhat newer gen 2 units.
If you do not know the password, you will need to set a PC to connect to tenor’s console port with hyperterminal (38400, N, 8, 1 None), then reset the tenor so that you can access the boot menu (when you see the “hit enter within 3 seconds” message, hit enter to access). From there you can set/change the IP address, subnet and default gateway. You can also reset the unit to factory default, erasing the config and password, but not the IP information you enter. This procedure works for all Gen 1 Tenors and only Gen 2 tenors that have software P103 or greater on it.