Ubuntu 14.04 was released on 17 April 2014, and today it is already the 18th.

Why isn't the distribution showing up in my ubuntu update-manager?

(I have set it so it will anounce every new version)

The content of /etc/update-manager/meta-release contains a link to: http://changelogs.ubuntu.com/meta-release-lts

Is it correct, that it doesn't show trusty yet?

So the only solution I see for me would be to set it manually with:

source /etc/lsb-release

sudo cp /etc/apt/sources.list /etc/apt/sources.list.$DISTRIB_CODENAME 

sudo sed -i s/$DISTRIB_CODENAME/trusty/g /etc/apt/sources.list

and then upgrade.

Do I have to disable the partner repos in this case?

#deb http://archive.canonical.com/ubuntu trusty partner