First remove 4 bolts or screws attaching the tilter to the bottom of the seat cushion
Choose either method
Removing plastic insert
Tip the chair over and insert a pole into the bottom of the chair base until it reaches something solid. Then, strike the pole end hard with a steel hammer until either the plastic insert along with old tilter removes or only the tilter removes. This may cause the plastic insert to break or only break it's holding clips.
Install plastic insert
Installing a plastic insert is not difficult. First place the insert small end into the top of the tube and press down until the larger part rests on the tube top. Some inserts may require a little persuasion by tapping in with a steel hammer. In order to avoid insert damage, place a small piece of thin wood between the plastic insert and the hammer.
Lastly, Install Tilter
Push the steel tilter spindle down into the plastic insert until the 2 clips engage and snap into the groove.
It is a good idea to replace the plastic insert when purchasing a tilter, since if the plastic insert breaks, becomes loose or has worn, the chair will be loose, rocky and wobble.