I have been running Windows 10 for at least a year on my 640. I prefer it to 8.1. It's about the same in terms of glitchyness. Messenger works well, but is painfully slow to load. Should you decide to upgrade, you would need to download the "Windows Upgrade Advisor" from the store. Once you run the app, it will confirm your phone's eligibility to upgrade, and green light the upgrade with the normal upgrade process under your phone's settings menu.