If you're on Windows, Linux or Mac you can download the Itch app and it will update the game automatically for you.
If you're on Android or don't want to install the Itch app then you have to download the entire update file.
For Win/Linux/Mac: Download the update, extract to a fresh folder, delete the old folder and play!
For Android: Download the update, extract if necessary and install on top of the old game (DO NOT DELETE THE GAME!)
Hope that helped :)