First you must say what type as there are differences depending on the mfg and model.Be the following basics apply.
1- Put the stem in the setting position (usually pulled out if stem set).
2 Them open cover to expose the movement. ( Screw off or snap open).
3- Remove the movement (Usually held in place with 2 screws at the outer edge.
4-Then if there is a screw in the part that holds the stem loosen that.
5- The stem can be held with a pair of pliers on the flat and the crown unscrewed. Be careful not to damage the square (FLATS), with the pliers. A rag or other safety measure should be taken.
6- then inside the stem hole, looking from the ouside the is an annular area around the stem that has a ferrule screwed into the case this has to be unscrewed and will bring out the stem and the ferrule.
This siounds complicated but the steps are more involved to write than they are to perform.
There are many variations to this process, and each type should be understood before proceeding.