When you click on a page to edit it, you can choose the Position from the drop down:
- Before tabs at top (i.e. before Questions/Tags)
- After tabs at top (i.e. after Ask Question/Admin)
- Far end of tabs at top (i.e. aligned to the far right side, it will be the leftmost of all the tabs on this side)
- After "PageA" (i.e. just to the right of Page A)
There is no way to automatically arrange them alphabetically but you can do this manually by changing the Position value.