Volvo V90 2018 Owner's Manual Supplement page 85

Press the booster cushion rearward to lock it
in position.
Upper position (assuming the booster cushion is
already in the lower position):
Press the button to release the booster
cushion.
IMPORTANT INFORMATION
Lift the front edge of the booster cushion
and press it rearward toward the backrest to
lock it in position.
* Option/accessory.
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