OLPCUpdate

[edit] Clean Install

This will COMPLETELY WIPE the data from the XO... YOU HAVE BEEN WARNED

download the latest XO OS build (os###.img and os###.img.fs.zip) to a thumbdrive
rename os###.img.fs.zip to fs.zip
insert the thumbdrive in the XO and power off the unit
while pressing all four game buttons (the cluster of four buttons above the power button) power on the machine
follow the directions on the screen