Read the Changelog.
You can download F.O.X. from SourceForge (you will be directed to a local mirror). Note that the Fox version will be automatically selected for your platform (Linux, windows, MacOS)
Then follow the installation instructions.
Note that to get Beta versions, you should browse the full list of available files, and select the platform that you are interested in.
There are available, depending on you platform and method of installation:
Fox-(version).tar.bz2 : Fox for Linux (source code)
Fox-(version).dmg : Fox, precompiled for MacOS X - a Universal binary (powerpc and x86) is available
Fox-(version).zip : Fox, precompiled for windows
FOX Wiki