I have never fixed this bike but normally there is an allen wrench bolt right on top of the shaft you have to loosen it (not take it out there is a tightening nut at the other end of the that you want to stay on) then after it is lose you should be able to adjust it but if not you will have to loosen or take of the first bolt down the shaft and that should make it able to be adjusted i hope this helps
SabiousOne