Updating
To update the game from itch.io, download the new version the same way you downloaded the game before.
If you're logged in and you bought the game here, it will say "You own this game" at the top of the page. Click the download button right below that and it'll take you to your download page.
Download the game, extract the files, and replace the old game files on your computer with the new ones.
If you want the game to update automatically, use the Steam key on the download page and add the game to Steam, or use the itch.io app.