FWIW, this post is for an older version of WordPress and probably not relevant to most current installations. Nowadays (current version as of writing is 3.9.1), it is a simple case of drag and drop.
First of all login to your WordPress admin area, click on “pages” (left menu) then click the title of the page you would like to edit. You should get something similar to this:
Click on one of the red highlighted buttons to open up the gallery editor. Note if you click on the top one you will then have to choose the “gallery” tab in the pop-up. The gallery editor should look something like this:
Now you are in the gallery area you will see a list of all your lovely images and under actions is where you will want to put in the numbers for your gallery order. Make sure “order images by: Menu order” is selected and be sure to save your changes. Things like “ascending and descending” I think are self explanatory.
Hope this helps