Are you trying through apt-get?
I just did this today and from what I noticed it didn’t want to upgrade to snapshot version, because apt-get was considering it a downgrade.
Some good info is in this topic openHAB2 snapshot on Raspberry
In short
given that you have correct reference in apt-get sources, this is the command that worked for me:
sudo apt-get --reinstall install openhab2-offline=2.0.0~SNAPSHOT
or for online version:
sudo apt-get --reinstall install openhab2-online=2.0.0~SNAPSHOT
You also can see all possible versions by running (it should show both beta and snapshot):
sudo apt-cache policy openhab2-offline