In terminal run:
esudo epad /etc/apt/sources.list
Towards the bottom of the file, you will find the entry for the Bodhi repo. Add the word "unstable" to the end of this line so it looks like:
deb http://packages.bodhilinux.com/bodhi trusty main unstable
Save and close the file. Run your system updates and you are now on the unstable branch! Please remember to report bugs and expect occasional hicups.