Your save files are in a separate folder, so you can just download the new version and it will have your old saves. You can also use the app to update the game automatically: https://itch.io/app
Your save files are here, you can make a backup just in case:
Windows: C:\Users\*username*\AppData\LocalLow\Glitch Pitch\Idol Manager\data
Mac: ~/Library/Application Support/Glitch Pitch/Idol Manager/data/