If it works like the White treadle I have, once the machine is in the upright position, cabinet open, there is a button or small lever on the bed of the machine. This is in front of the pillar, near the serial number, right side if you are facing the machine as if to sew. Use this to release the machine from the cabinet and tip it backwards. When you do this, there are little screws that hold the hinge posts at the back of the machine. Loosen these (but don't totally remove to keep from losing them) and your machine should pull up off the hinges. The hinges will still be attached to the cabinet. I don't have my treadle accessible right now, but this is how I remember it working. Hopefully someone else will pop in and verify or give some better instructions.
