Note: Depending on where you live, certain channels are illegal to use, but if you say, move or buy a used router from out of country, this could be usefull. Also I don't think the Gov't is going to show up if you use channel 13 in your basement, but you have been warned!
Also Note: Some WiFi NIC's are blocked from Accessing channels, so even if you unlock channel 13, your IPad may not connect to it!
First Enable SSH & login via SSH OR Run Via
Administration >> Commands
Then issue the following command:
[EDIT] DD-WRT v3 now has this option built into GUI!