I'm afraid that's currently not possible. Bones on the same hierarchy level are sorted in alphabetical order. You could prefix the bones with appropriate letters if you want to change their order in the hierarchy.

Hello! Yes you can reorder bones in the hierarchy, by dragging and dropping them under different bones, or pressing P then selecting a new bone.

It is a bit weird however to want the head bone to be the top of the hierarchy, as it would also be the pivot of all movement. Usually, the head is parented to the neck, which is parented to the body, parented to the hips, parented to the root, and not the reverse. You can check out the example projects that come with the editor to get a better idea of how bone animation works. I also suggest giving a go to these resources:

User guide and user guide videos. An amazing and complete start on the software:
Getting Started - Spine User Guide

Example pages with explanations of the projects:
Spine: Example Projects

Long but complete in real-time rigging sessions with downloadable projects:
Spine: Twitch

If you meant visually on top though, then the bone drawn last in the viewport will always be the one most on top, no way to reorder it. It generally doesn't bother anyone, but I thought I'd mention it just in case (: