If I'm following this correctly, it looks like the Bone you want to use as root is already a child of the next bone, so you will have make a couple changes in the hierarchy:
- You need to be on frame 0 (unless what you want is to animate the bone hierarchy)
- Select the bone you want to use as root, now select the Reparenting Bone tool and click on any empty space in the workspace
- That will make that root bone to have no parent
- With the Select Bone tool, select the bone of the body
- Now select the Reparenting Bone tool again and click on the root bone
- An arrow will go from the body bone to the root. That means the hierarchy will be correct now