Disclaimer:

(DO NOT USE ONLINE, Change your game to single player only in the main menu) - 
If you ignore this fact and go into an online game, you may be banned from playing online within the game FOREVER.


Guide:

Go To : C:\Program Files (x86)\Steam\steamapps\common\Dying Light\DW
Replace "Data3.pak" with the one found in the Rar file

(If your game is already modded this way then you probably know what you should do)