Determine where you will cut the box spring in half. It should be just to one side of the middle crosspiece. Remove the staples holding down the mattress cover, just for a few inches on either side of this mark. You will need to pull the mattress cover back to fold the box spring in half, and the cover would keep you from being able to do that. You don’t have to take it all the way off; you just have to be able to push it out of the way.
Using a handsaw, cut through the frame JUST to the left or right of the middle crosspiece. Do NOT cut any crosspieces! Repeat on the opposite side and the middle, like this:
Set the box spring on your bed frame, add the mattress and bedding, and enjoy your new bed!