Usually, the seat bottom cushion of the backseat has to come out first. In most cars, it's usually bolted in place at the bottom outside front corners or it is hooked underneath in the front by brackets welded to the floor that engage to spots in the seat frame. The bolts or retainer brackets can be several inches inboard from the outer edge so feel around that area. Try pushing down and backwards on each corner of the seat cushion and then lift upwards to see if it will release from the brackets...or check for bolts at the bottom front corners.
Once the seat cushion is out, the seat back usually has bolts in the lower corners that hold it in...remove those and lift or tilt up on the seat back to remove it. There's usually a ridge or rim at the top.. back side.. of the seat back that engages a metal strip to hold it in place. Reverse the procedure to reinstall.
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