Look at this: https://github.com/gitlabhq/gitlabhq/tree/master/doc/update
This how to's explains about upgrading the gitlab. But there are certain concerns.
Gitlab can be upgraded but gitlab-shell throws an error as "fatal: Not a git repository (or any of the parent directories): .git" which I guess needs answers from "Bitnami" or from Guru's.
So, be cautious.