How much do you want to remove? If you are just wanting to reupholster the cover or replace the foam center you should lower the seat back so that the two seats backs aren't next to one another. With a screw driver pop the plastic cover off the shaft end and remove the large four groved nut. It takes a special tool I suppose but we have never needed it.
If you want to totally remove the armrest for good you probably want the shaft removed too. To do this remove the back cover of the seat. One screw at the bottom and the cover slides down. You will need the seat back all the way up for this.
Once the cover is off the armrest shaft is available for remove. I don't remember how it held on but its not complicated.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician