I'll start the way the guide does: with caveats. Therefore, I must say you should only attempt this if a.) you know what you're doing and b.) it's legal and c.) you've read through all the caveats at ECAFG and understand what you're getting into. OK, that should make the lawyers happy (and stop you from becoming unhappy).
The guide describes a step-by-step method to change the settings to the speed controller. You'll need a laptop (PC, I think), some cables, a free file from ECAFG and a version of the GE Sentry Software ($40). Once you've collected these things, it's a fairly straightforward process of hooking the laptop to the car and changing the settings. The full details are available here.
[Source: Electric Cars Are For Girls]