Find the current version of the Enhanced Edition patch (=1.4) HERE. You also need an compatible language pack - download one or more from HERE. Make sure TWEE_Upgrade.exe and the language pack are in the same folder. Run TWEE_Upgrade.exe . It may take up to 5 hours to update.
Please note! The patch should NOT require you to register. If it does it's an old version of the patch, which won't work and you have to download the versions linked above.
An alternative solution to patching the older versions of The Witcher is to claim the digital backup copy of the game on gog.com/witcherbackup (more info available HERE). The gog.com version of The Witcher is fully updated from the very moment it is installed and doesn't need to be patched.