How to update to latest version

I've tried this before, and the assumption made was that my issue was resolved and my previous thread closed. I will attempt to explain my situation better this time.
I have installed Appointments+ version 1.0.9 on my site. I now have the latest version, 1.1.2, as a zip file on my computer (as I have the previous version, 1.1.1, which I could not upload). When I attempt to upload the new version, the attempt fails.
How do I update this to the newer version? I have also deactivated the former version to upload the newer version, and that also fails.
Normally, updates for plugins are not this challenging for me, so I am curious to know what makes this one so much different?
Any assistance would be greatly appreciated.

James