Download it from the Factor web site. Lots of new stuff in this release: FFI callbacks, multi-window UI, improved online help, and many updates to the contributed libraries. Also for the first time a pre-compiled Mac OS X (PowerPC) binary package is available! Just double-click on Factor.app and the Factor UI is started, no fuss.
Note that the detailed changelog is now part of the online help system, and so you can view it either from the Factor UI, or in the online documentation responder (as soon as I update the server on factorcode.org to 0.81, anyway).