The file tree is a click and drag interface, so to move a file, left click and hold on the file you want to move. While holding the mouse button down, drag the file up or down in the tree to where you want it.
To move a file between two other files:
Click and hold on the file you wish to move.
When you move it up or down the tree, the selected page will become a gray box.
When the box goes between files, a black line will appear.
When the black line is where you want the file to go, let go of the mouse button.
The file name will reappear.
In order for the change to take effect on the web site, the Generate Menus button in the Control Panel will need to be clicked.