I have never installed with apt-get, I used the source. But to update, I would do the following

1 Backup my database 2 Backup your openerp directory (wherever apt-get installed them) 3 Download the newest deb file from nightly 4 Upgrade with dpkg -i {package_name_from_nightly} 5 Start the Service to load the update (manually openerp-server.py --update=all) or update your init.d/openerp to add the --update=all flag

If this break, you can have to restore you openerp directory first, then restore your database. I would try all of this stuff on a test server first.

Hope this helps.