OwnCloud comes with its own online updater, similar to WordPress and Piwik. However, in the current version it is not enabled by default. To enable it, login to the OwnCloud webinterface, go to Settings > Apps > Updater and click Enable. You will now see an option Updater under Settings > Admin. It will either say ‘up to date’ when there are no updates available or show you the latest version that you can upgrade to.
Click on Update to install the latest version. Then log out and in again and the update will complete. This process of course requires that the files of your OwnCloud installation are writeable by the webserver. On many hosting packages this is already the case, if you run your own webserver you could change the permissions with e.g.
chown -R www-data:www-data owncloud
There are still a few issues with the updater, hence why it is not enabled by default. For example you will need to re-enable all apps and it can potentially break your database: http://apps.owncloud.com/content/show.php/?content=153278
You should also be aware that it creates a folder called backup and copies a backup of all owncloud scripts into it before starting the upgrade process. The problem with this is that the backup folder is publicly accessible by default. To solve this you can add another rewrite rule to your .htaccess file, for example
RewriteRule ^backup/(.*) index.php [L]
So all in all, the updater is still a beta version and a bit buggy, but it is already a huge time saver and helps to keep OwnCloud up-to-date.