You only need to buy once to get all the updates.
You may download and install itch app for auto-updates, https://itch.io/app. Itch app is available for windows, just like Steam.
If you are not using itch app, you may have to manually re-download the latest update everytime, then copy & paste the previous save file to the latest version of LonaRPG you just downloaded.
P.S: If Itch app isn't working on your desktop, it might've caused by antivirus. Try temporarily turning off antivirus then run Itch app again. Once Itch app has started up, you can turn on the antivirus again.