if you already bought the game, you should also have an email where you can dowload the game. when you press "download your purchase" you shoul see the new files. just download the file you need and start the game. :)
If you are on itch.io you will have in your library a list of "things you own" go look for that game there and click there the download button and you will see the new files
You also need to delete older versions and download the new game
