there is usually a small hex head bolt that is holding the shift knob on. if you can find that and unscrew it the shift knob will slip right off. if might be under a ring though for cosmetics so if you have any rings on it, usually chrome finished, you can see if they will spin to unscrew or if you can pop them off to get to the hex head bolt. you might just be overlooking it though because they are usually flush with the shift knob and the same color so look really close when you do try to find it.
if there isn't a bolt you might want to look for a c clip that is holding it on, once again it will be very well camouflaged with the shifter so you will have to look hard, once you find it, using a flathead screw driver, pop the clip out and the shift knob will slide right off.
good luck
