Are there any official guide for where to download openhab2 from and how to install? And as I understand Openhab2 is only at alpha state, how do I keep it updated?
Instructions for installing OH2 are here.
You can download the latest release here.
Or you can checkout and build from the nightly builds here.
It is Alpha and undergoing lots of changes and updates right now so there really is no automated way to keep updated. You have to manually pull the latest nightly build and/or watch for an announcement that a new release has been made here on the forum.