As previous answer , but a tip is......... when you loosen the center bolt, give it a tap ( the bolt ) not too hard, this makes the job a lot easier. This tip only applies for a 'quill' type stem not for the 'aheadset' type found on most new bikes.
Aidan
July 2005
There should be a bolt head on the clamp in the centre of the handlebars. This goes down the centre of the tube and ends in a wedge. Loosen it off and twist the bars while you pull upwards. Don't loosen it too far though, a couple of turns should do. If the bars haven't been adjusted for a long time you might have to spray wd 40 on the tube first.