This trick was included in the comments of a blog post

How to change the DHCP server start_ip. This is how to do it  with a telnet session.

rg_conf_print dev/br0/dhcps/start_ip (start_ip(192.168.1.2))
rg_conf_set dev/br0/dhcps/start_ip 192.168.1.20
rg_conf_print dev/br0/dhcps/start_ip (start_ip(192.168.1.20))
save

Alternatively, you can change the DHCP settings through the web interface.

Go to “Network interfaces” and click “Lan Bridge” . Further down on the page, click on “IP Address Distribution”. On the next screen, click on the “Action” icon, at the right side of the Lan Bridge entry.

There you are.  Change the settings, if you know what you are doing of course.