If you set up a folder for it to download into, the old versions should be kept in there until you delete them. (If you've ever noticed an Itch.io game give you multiple files to choose from when you start a game, that's why) If you're on the app, you can click the gear next to the launch button, click "Manage" and then "Open folder in Explorer" and you should be able to see all of the versions you have downloaded, and even launch a specific version you have directly from Explorer if you want.