Undo the bolts on the stem.
On Ahead stems, there are two allen bolts which clamp the bar stem to the steerer - just loosen them- located on the back of the stem (facing you as you sit on the bike).
On older standard headsets, just lossen the allen bolt where the stem goes into the steerer !
Old headsets - the stem goes into the steerer, aheadsets, the stem clamps onto the steerer. With old headsets you need two big spanners to adjust them, on aheadsets you just use the top allen bolt on the stem to tighten.