First of all; take a backup copy of your current installation, including the database!
Shut down your Tomcat server
Again, make sure that you have made a backup of your Tomcat webapp (all files) and the database!
Go to the directory 'WEB-INF/lib' and remove the file 'ical4j-1.0-rc2.jar'.
Extract the content from 'vp_installed_v6.4_upgrade_from_v6.3-20101107.zip' over your current installation.
Update your VisionProject database by running the SQL script 'upgrade_from_v6.3_to_v6.4.sql' located in the directory: '/WEB-INF/internal_docs/installed version/db_scripts/v6.4/Upgrade from v6.3'. Choose the directory that match your database, MS SQL Server or MySQL.
You are done and can start Tomcat again!