With the classical stems there are two versions:

- slanting wedge: Technically the better solution. A slanting steel wedge is pulled toward the accordingly slanting stem by a bolt running within the stem. This does create a quite big clamping area between steem and the inside of the fork shaft.

- chuck cone: There a conical formed steel part is pulled inside the hollow stem by a bolt. This does lead to a strutting of the stem and does create a ringlike clamping area.

To loosen this stems the clamping part must be pushed downwards for both versions.
Loosen the bolt (6 mm allen key or 1/2" hexagonal head), ending at the top of the stem, for 3 to 4 turns. The stem will still be fixed.

To get the clamping loose you have to push the bolt down. Use a soft-head-hammer (or hammer and wooden pad) and set a well hearted blow on the bolt. As you will need some force make sure that the fork is well supported.

If there has been used plenty of grease while fitting, the stem can now be removed upwards easily.

If corrosion between stem and fork shaft occured (most likely with aluminium stems and steel shafts) you will have to try to get it loose by turning and pulling at once. This does require two persons - one that grabbs the stem (best with handlebar attached) and another one to rotate the fork. Turn the fork slowly to the left and right and pull continously.

Sometime using penetrating oil for at least 24 h and from both sides may help somewhat.

In dogged cases using cooling spray can help. To do so remove the bolt and apply cooling spray inside the stem shaft using a suitable tube. Normally this treatment must be repeated several times. While cooling the inside simultanously pull and twist handlebar and fork.  Of cause there are 2 persons required and to get a better lever you may use a wooden latch below the forks head.