Everything works now. Thank you.
I just went ahead and downloaded sendy-5.5.2.zip fresh from the link in the Upgrade email I received.
I can't actually access Settings to get my new License Key. I assume the same problem that makes the Edit and Duplicate button unclickable is making Settings unclickable.
Thanks for your reply. Is dragging the folders into my root Sendy FTP folder sufficient? The new folders will overwrite the old? Or do I have to open each folder one by one and drag in the new files?