there are 4 screws that hold the box to the floor. remove the black knobs, and the top access cover. the screws are at the four corners.
soak the valve with a penetrating oil, and let it sit for a while. use a 7/16 socket to turn the bolts. they will turn if you put enough pressure on them.
take the not off, and drive the bolt out with a hammer. that works for me sometimes when they are stuck.