The Prime User Guide doesn't seem to mention that you can rearrange the "Fixed" sort order of the icons in the Apps Library. The following example will swap the first two icons' positions, but you can use the same method to rearrange all the icons.
First, press Apps, Sort, and select Fixed. You'll see this familiar sight:
Now press the Function icon and hold it down steadily for one full second. Arrows will appear in the corners of that icon:
Now DRAG the icon to the right:
Now let go.
Done! Your new customized arrangement of the "Fixed" sort will remain "fixed" that way until you change it again.
Rearranging many icons gets kinda fun; it reminds me of those sliding tile puzzles we all played with as kids.
Practical use: The icons for six built-in apps and all your custom apps are offscreen when you press Apps in "Fixed" sort order. This trick lets you move your favorite ones up so that they are available immediately.