This assumes you have wikipbx 0.8.0 or later installed in /usr/src/wikipbx, so that your settings file is /usr/src/wikipbx/wikipbx/settings.py
The only file you should need to save is the settings.py. Everything else is just code and can be replaces with the newer code. All the rest of the configuration data is stored in the database and therefore does not need to be touched.
$ cd /usr/src $ mv wikipbx /tmp/wikipbx.OLDBACKUP $ bzr co lp:~wikipbx-dev/wikipbx/0.8.x wikipbx $ cp /tmp/wikipbx.OLDBACKUP/wikipbx/settings.py wikipbx/wikipbx