1. Navigate to your game folder by right clicking your game in the library and selecting Manage->Browse local files. You can also go to C:\Program Files (x86)\Steam\steamapps\common\Dying Light 2\.
2. Go inside the ph folder, then inside the source folder. Drop "Data8.pak" there, done.
3. If you are using mods and already have Data8.pak there just rename it to Data9.pak an so on.

Credits: xowny