itch.io

I am pretty sure you should just be able to go to your library tab and see it in there and then download it from there for the new update. If not, click I already bought this in the window when you're purchasing I think, and you should be able to get an email with all of the links of the games you have bought in whichever email is linked to the account that bought the game.

...You just redownload it. There's a link at the top of the page.

It's on you for not actually looking at the page.

GOSH DANGIT LOL
Itch is so confusing. If you need a refund just ask. 

You should be able to find every new version in the itch.io library.