I had a horrible time downloading the new team app, even before I switched to Windows 10. I eventually uninstalled the old Team Time Tracker and installed the new. No problems at all since I switched to Windows 10. Don't panic. If all else fails, just delete the old tracker and then install the new. Worked for me.
I've downloaded it since its release and havent really noticed anything strange or different except that when I checked my Anti-Virus program, I noticed it was blocking the Upwork app. Never happened on 8 so I had to manually override the block by clicking on allow.
Anyway, everything is working fine, no issues. They even made a small update for the Upwork time app, which I believe was to ensure compatibility with win10.
I have not downloaded Windows 10 yet, but have my laptop that I don't use for work set to update to Windows 10 so I can play with it before I update my work laptop and desktop, but I am happy to see it has been updated, because once I have played with it and decided if I like it or not would be updating. And eventually Microsoft will do what it has done in the past and stop supporting a device which we will all have to update eventually.
I'm running windows 10 and have not run into any issues so far with the time tracker. The messaging system that comes with it doesn't work that well but it also doesn't work that great for me on the website as well either.
Sometimes the ap disappears from you toolbar. Click on the windows logo (bottom left), go type in Upwork in the space with the looking glass...click on the ap icon...and voila...