The app checks every hour for a new update automatically. However, if you choose “Check for updates” in the menu bar, we will basically deliver you the latest version in that list. We will show the "Updating..." text when our third party service thinks there is an actual update in progress, so it's not clear if that state is not getting updated correctly, or if the update is hanging forever.

If your menu bar is stuck at "Updating...", quit and re-open Rewind and you'll be able to click on "Check for Updates" once again.