Whoa! You guys have been busy this morning. Let me update you and then I'll read through all of your posts.
Not sure why the links were not working for the update, buuuut I did something odd and was able to download it. I copied the link url and pasted it into another window. Ran the update and got something after a long arse wait saying that it couldn't perform making the update.
But just before coming here to update everyone, I had an idea to test. I right-clicked an epub file of some top seller's book and asked it be opened with Sigil. Although, I can still do nothing and have the exact same symptom, Sigil did pop up with that book loaded. Have a look. Not sure if that's helpful or not.