First make a backup in case of accidents (unlikely but possible). See the
FAQ for help.
Now find the installer for the English version of the game. If you can't find it, redownload it from
http://www.ldwsoftware.com/zips/VV3Setup_v10004.exe Run the installer. (Double-click on the file.)
When you reopen the game it should be in English again.
Note that this only applies to games downloaded from the LDW site.