If your primary tab is locked, go to Admin > Company Settings > Company Page Settings. Click on the arrow next ot the primary tab name to see the sub pages. Then drag and drop the sub pages in the order you want them in. 

If your Primary tab is not locked, then click on the Manage Pages icon on the primary tabs bar(very right). look for the name of the primary tab and expand it and reorder the sub pages by dragging and dropping them in the order you want. 

Also, keep in mind that by default, the subpages will appear in the order you share them with others.  So if you haven't locked them down and you share them out of order, the reciever of shared pages will have them in the incorrect order.