The auto update doesn't get enabled until everything has been stable for a while. Upgrades are nearly always more tricky than a fresh install - so many addons and existing content that could be affected.

To upgrade an existing site you will need to manually upgrade. The procedure is described in:
http://www.concrete5.org/documentation/how-tos/developers/manually-...
There are also some notes within the zip file about upgrading and manually installing.

Don't forget to make a full database and files backup first. If the upgrade fails part way through or turns out to be incompatible with something on your site, there is no going back.

Many (including me) prefer to make a clone of a site and test the upgrade on that first.

You may find Backup voodoo of interest.
http://www.concrete5.org/marketplace/addons/backup-voodoo/...