Did you install with git clone git://github.com/CleverRaven/Cataclysm-DDA.git?
Then “git pull” from inside the directory (the one created by “git clone”) should bring it up to date. It will already be up to date after cloning, so “git pull” will do nothing until someone commits some actual changes.
The git version is bleeding-edge, so even more unstable than the nightly version, but very easy to update once you have it set up correctly.