When you add pages or files to a folder, they will be added to the left hand navigation in the order that you add them. When you want to change the order, navigate to the top level of the folder and go to the content tab on the editing bar - you will then see a table listing the content of the folder, and down the left side of the table there is an icon made up of dots which you click and drag then release to move items up and down. When you click and drag the line turns yellow:
You will not see the changed order in the left hand navigation until you go back in ‘view’ in the editing bar.
Removing from navigation
If you want to remove items from the left navigation (especially pdfs or other added files), select the item then the 'edit' tab in the editing bar, then select 'Settings' from the sub navigation. You can then check the 'Exclude from navigation' box and save and the item will disappear from the left navigation:
You can bring the item back into navigation by reversing the process.