First of all; take a backup copy of your current installation, including the database!
Again, make sure that you have made a backup of your Tomcat webapp (all files) and the database!
Extract the content from 'vp_installed_v5.3_upgrade_from_v5.2.1-20091118.zip" over your current installation.
Update your VisionProject database by running the SQL script 'upgrade_from_v5.2_to_v5.3.sql' located in the directory: '\WEB-INF\internal_docs\installed version\db_scripts\v5.3\Upgrade from v5.2\'. Choose the directory that match your database, MS SQL Server or MySQL.
You are done and can start Tomcat again!
NOTE! If you like to use German language you have to add 'de_DE' to the the availableLocales attribute in the file: 'VisionProject.properties', like this:
You can also set German to be the default language on the login page with this:
Then you have to set the Default locale in 'Settings -> General' to German.