Depending on the model it is usually held by 2 phillips head screws which are screwed vertically thru the parcel shelf from the boot into the seat back/top. Once these are removed you will also find that the top of the seat is "clipped" over a small metal lip on the parcel shelf. This requires you to lift up the top of the seat until it is free of the parcel shelf and then swing the back seat forward to remove the 2 small wire pegs which are in holes into the bottom of the boot behind the bottom of the back seat.
Quite easy once you have done it a few times.
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1969 Cooper "S"
1967 Mini Deluxe
1973 Clubman Van (the fleet spare)
1978 ex 1275 LS ("Wizard" Eaton Supercharged)

2015 HSV GenF GTS(occasional drive & tow car)
2019 MINI F55 Cooper S