When you're editing a handbook, the navigation / table of contents is displayed as abstract bars on the left side of the screen:
This is so it's not a distraction and you can focus on the content on the screen. If you hover over the bars, you'll see the actual text:
To move any section, just click the section you want to move within this view, hold the click, and drag up or down. Once you release the mouse button to drop the section, your view will update to reflect the new order of your sections.
Note: Be careful when moving an entire chapter. Currently you can only move one section at a time. This means moving an entire chapter full of sections can be a chore. It's also very easy to lose your place. If you need help, please contact us. We do plan on improving the way this works at some point in the future!