Hold down the reset button for 10 seconds. The button is located next to the power jack. The device will reboot when you release the reset button.
Set your computers IP address to 192.168.168.167 MASK 255.255.255.0.
Connect your computer to the LAN port on the SonicWALL TZ.
After the device reboots it will be in recovery mode. Connect to at at http://192.168.168.168.
Click the boot icon next to “Current Firmware with Factory Default Settings.”
After the device reboots, you can again connect to it at the 192.168.168.168 address. The login is: admin and password.
At this point the device is reset to factory defaults.

How to factory reset a SonicWALL TZ firewall