There is no way to alphabetically sort these automatically built into Word.

You can use the Navigation Pane or Outline view to rearrange them yourself though.
Moving/Reorganizing Pages in Microsoft Word

It may be possible to use Outline view showing only the headings and run a sort. I have not tried this but it may be something that can be done. Try using a copy of your document. To do this, you would need to add the Sort command to your QAT.