Computer chairs come in various shapes and sizes. Gear Patrol suggests that you purchase a chair with at least five adjustable features. In addition to the height adjustment, you should be able to adjust the arm height and the back support. The height adjustment is controlled by a gas cylinder that allows the seat of the chair to move up and down. If your seat won't stay in position or if it moves randomly, the gas cylinder is faulty and should be replaced.
Place the chair in a horizontal position so that it is lying on its side.
Remove the base of the chair. The base is held in place either with retaining clips or with nuts and bolts at its centre. Remove the retaining clips or unscrew the nuts and bolts with a wrench and screwdriver.
Place a pipe wrench on the gas cylinder. Spray the gas cylinder with WD-40 to lubricate it and make it easier to remove. Twist and pull the wrench to loosen the cylinder and remove it from the column.
Take the old gas cylinder to the office supply store and purchase a new one. The new cylinder must match the old one exactly to fit your chair.
Install the new cylinder. Follow the installation instructions for the cylinder since the specific details for each cylinder may be slightly different. The general idea is to place the tapered part of the cylinder into the column and rotate it into place.
Reconnect the base of the chair. Set the chair in the upright position and adjust the chair to the desired height. Sit in the chair to test it. Check it a few hours later to make sure the seat has not lowered.