If you have a lot of pages in submenus, reordering them can be tricky. Certain pages may link to other pages, and changing the page order could impact those links.
Using the page hierarchy Page 1 > submenu 1 > subpage 1 + subpage 2 as an example, it's important to note that subpage 1 cannot be dragged and dropped directly into another submenu. For example, here's what not to do:
This article explains how to move these pages.
Go to the Pages tab.
Drag the subpage you want to move to the bottom of its submenu.
Drag the subpage into another submenu.