You would need to change /etc/apt/sources.list.
You will see a line that says
deb http://apt.osmc.tv jessie main
Changing it to
deb http://apt.osmc.tv jessie-devel main
will make apt-get and the built in updater use the staging repository.
I do not recommend you do this at this time however, and we cannot offer any support when using the staging repository.
Sam