View this online tutorial for a how to on changing your LAN IP address and/or DHCP server settings.
If you already have a DHCP server on your LAN network DO NOT connect the m0n0wall to your LAN network until you have first disabled the DHCP server on the m0n0wall. Otherwise your m0n0wall might respond to a DHCP client computer on your LAN before your normal server and thereby give incorrect information to the requesting computer.