You'll need DesktopX, I'm not sure if the trial version is enough. I use the standard version.
I posted this on the steam forums, might as well re-post here.
It's actually really simple, open a file like MainGameWnd.dxpack and you'll instantly recognize the main strategic UI.
As for editing them, make sure that DesktopX is set to start in builder mode. You do this by loading a .dxpack, then rightclicking the icon in the right part of windows' taskbar and selecting Switch to DesktopX Builder. After that, the Builder is loaded and you can set it up in its settings so that the Builder is default loader.
Once you're in the builder, you can basically move things around however you wish. Keep in mind some UI pieces have "scripts" that may override your modifications. You can also put your own basic positioninig scripts into the UI (such as "center in middle of screen"). To "execute" all scripts, open up a script for a UI piece and close it again.
Right-click the taskbar icon again and go to "List Objects". It will greatly help with fine details and gives you overviews of scripts and such. When ready to export, rightclick in list (Select->All) then right-click (Export). Overwrite the file you loaded (save a backup first!).
This might sound like a mouthful but really it's a 3 second procedure once you know what to do.
Edit: Oh yeah, one more thing. The "scripts" function in DesktopX, but they are actually mirrored to the "comments" of each UI piece. FE reads the "comments" script, NOT the "scripts" script. It'll make sense when you see it.