There is a lot of force exerted on the stem when you ride and if the stem is too high, there is always the possibility that the stem could break if you should happpen to hit a pothole, a big bump or a curb. If you have the stem as high as it can go and it's still not as high as you would like, there are longer stems available for most bikes. Don't cheat.
Fig 1) Locked position
The stem bolt is tight and holding the expander/wedge up into the base of the stem forcing the bottom of the stem to be wedged against the inside wall of the fork.
Fig 2) Intermediate position
The stem bolt has been loosened (ccw) leaving a gap between the head and the top of the stem. The expander/wedge is still in place so the stem is still being held in a locked position.
Fig 3) Released position
The expander has been released by tapping down on the head of the stem bolt so the bottom of the stem is no longer wedged against the fork. The stem is now loose so it can be removed or repositioned.
2) Use a rubber mallet to tap the stem bolt back down to free the expander or wedge plug. You may have to give it a few good whacks if your stem hasn't been moved in a while. If you don't have a rubber mallet, you can put a piece of hardwood or a thick piece of rubber on the head and use a regular hammer.
3) Position the stem where you want it and tighten the stem bolt as tight as you can get it. Don't over tighten it by using an extension on the wrench, just make it's as snug as you can get it.Note If you are raising your stem, make sure that you don't raise it too far. There is usually a line on the stem indicating the maximum height you can have the stem at. If not, the rule of thumb is to have the top slots in the side of the bottom of the stem (wedge type stem) or the highest point of the slant at the bottom of the stem on the wedge type stems at least an inch inside the fork (see fig.4).