Any time you want to upgrade CKAN to a newer version, first back up your database:
paster --plugin=ckan db dump demo_ckan_backup.pg_dump --config=demo.ckan.net.ini
Then run this as root:
apt-get update apt-get dist-upgrade ckan-std-install
You need to use dist-upgrade rather than upgrade because it is possible more recent versions of CKAN will add new dependencies rather than just updating existing ones, and apt will only install new dependencies if you use dist-upgrade.
The ckan-std-install will also upgrade your existing CKAN instance, keeping a backup of your old CKAN config file.