It should be pretty easy. First you'd need to install Fluxbox. At the command line:
I'm assuming it's in Debian's ARM repos, if it's not you might need to compile it.
After that, edit the ".xsession" file in your home folder (/root if you're logged in as root (which isn't a good idea usually) or /home/<username> otherwise). You might need to create it if it dosen't exist. Make a copy of it if it does exist, so you can switch back to LXDE easily. If the text "lxde-session" appears in the file, replace it with "fluxbox". Otherwise, add "fluxbox" to the end of the file.
After that, just start up X as usual, it
should start up in Fluxbox.