I have a snapshot that has been stuck in the uploading stage for over a month. How do I 'force quit' the current snapshot process?
Once you go to the "Snapshot -> Managed Backups" page in your site's back-end, you should see a list of backups there. Next to each one there's a "three dots" icon (on the right) so click on it for the backup in question and there should be a "Delete" option. Such backup cannot be "unstuck" but you should be able to delete it.
That being said, is it just one backup that "got stuck" or you weren't/aren't able to create Managed Backups successfully (with upload to the cloud complete) at all?
I also noticed that you seem to be using version 3.1.7 of the Snapshot Pro plugin. It's already a bit outdated release and since then there were some important fixes added. I would strongly recommend updating it to the most recent version. Actually, I think it would be worth to start with it as it might even "un-stuck" that backup too - so it's worth giving a try.