This should be what you're looking for (from System Manager Help):
Reset a non e-Series device manually
Turn the Firebox off. On the front of the Firebox, find and press the
up arrow button.
Hold down the up arrow button while you turn on the Firebox, and
continue to hold the button down until the LCD display shows the
Firebox is running in safe mode. When the Firebox runs in safe mode,
it is running in factory-default mode. In factory-default mode, the
Firebox trusted interface is set to 10.0.1.1.
Connect a cross-over Ethernet network cable between your WatchGuard
management station and the trusted interface of the Firebox. The
trusted interface is labeled interface 1 on the Firebox. Change the
IP address on your management station to 10.0.1.2 (or another IP
address from which you can connect to the Firebox trusted interface at
10.0.1.1/24). If your management station uses Windows XP: From your Windows Start menu, select Control Panel > Network Connections > Local
Area Connections. Click Properties. Select Internet Protocol (TCP/IP)
and click Properties. We recommend that you ping the trusted interface
from your management station to make sure you have an operational
network connection. Open Policy Manager. You can open an existing
configuration file, or create a new configuration file. Use the
options available from the File drop-down menu. Select Setup >
Feature Keys. Click Add and paste a copy of your feature key in the
text box, if necessary. You can also import a Firebox feature key by
clicking Import.
When you are ready, select File > Save > To Firebox. Save your
configuration to the Firebox at IP address 10.0.1.1, with the
administrative passphrase “admin”. After the Firebox restarts with
its new configuration, we recommend that you change the passphrases
for the Firebox. Select File > Change Passphrases to set new
passphrases. You can now put the Firebox back on to your network and
connect to it with the IP addresses and passphrases you set in your
new configuration.
If you did not change the IP address or passphrase, you can connect to
the trusted IP address 10.0.1.1 with the passphrase “admin”.