I have used official documentation to install the community version (https://www.odoo.com/documentation/9.0/setup/install.html#setup-install-source). The problem is that when I try to update using apt upgrade odoo it says it is the latest version from 07-07-2018. However, it is not the latest. I tried to update it from Github but it was not possible:

fatal: 'origin' does not appear to be a git repository
fatal: Could not read from remote repository.