First, go to your Edit Comic area. You can reach this by clicking on "EDIT" next to the comic in your user sidebar:
In your Edit Comic area, click on the "pages" button:
You'll see a list of your pages. If you have a lot of pages, then you'll only see about 10 or so at a time.
Use the little up and down arrows to move the page up or down by one.
Currently there is no way to move pages in large jumps, although that functionality is planned.