Look underneath the seat and you will find rails that allow the seat position to be changed. Inside the bottom rails that sit on the carpeting (called tracks sometimes) are bolts that bolt it to the floor of the car. Unbolt it and slide it out the door. Make sure you unplug any seat belt related switches and if it is a power seat, the wiring plug that connects power to it.
If you only want the upper seat and want to put another seat on the existing rails, it is still easier to do the steps outlined here and then flip the seat over outside of the car to remove any parts you want to put back into the car. I did that with van seats for a cabin once. Make good comfortable seats as long as you bolt the rails to something like some boards to make them less likely to tip over when someone leans back.
