To move a facet within a facet set:
In the Administration Options menu, go to Search Workbench > Customizations > Facet Sets > Your Facet Set.
Right-click the facet and select from the menu to copy or cut it.
Right-click the facet set or facet under which you want to paste the moved facet. You can paste the moved facet either at the same level as the selected facet, or as a child of that facet.