The top and bottom holders have different sized grooves to hold the seat rails.
Find the one with the smaller grooves (it should be at the top but it depends who installed it) and slide it out.
If it wont slide out due to being too tight then wedge something in the bracket/hinge area (screwdriver, bar, spanner etc, etc) and lever it so that it loosens the seat holders and then slide the smaller grooved seat holder out.
Do the reverse to get a seat back onto the post!!