1. Tap the  Thumbnails icon on the bottom navigation bar. If you don't see the navigation bar, tap the center of your screen to bring it up.

2. Long press on the page you want to move until it becomes editable. When the page becomes editable, it becomes larger and less opaque than the other pages, and you are able to drag it around.

3. Drag the page to a new position and release it. The page numbers will update automatically according to the new page order.