Toyota Prius Owner's Manual page 112

1-4. Adjustable components (seats, mirrors, steering wheel)
Folding down the seatbacks

Remove the seat belt from the guide

Returning rear seatbacks
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Fold the seatback down while
pushing the lock release button.
To return the rear seatbacks to
their original positions, lift them
up until they lock.
If the seat belt interferes with luggage on
the folded seats, remove the seat belt
from the guide.
Tilt the rear seatback up until it locks,
making sure that you hold the seat belt to
prevent it from getting caught between
the seatback and the inner side of the
vehicle.
If the seat belt is removed from the guide,
make sure the belt pass through the
guide.