Hi all,
To upgrade your Zentyal 3.3 Community Edition to 3.4 you only need to update your 3.3 first to get the new zentyal-core 3.3.8. After that, an upgrade button will appear on the dashboard.
Please take into account that this upgrade involves also a distribution upgrade (from Ubuntu 12.04 to 13.10), so it make take a bit longer depending on the packages you have installed and your internet connection.
Remember also that only one community edition is maintained at the same time, so the end of life of Zentyal 3.3 has been reached already.