Finally I upgraded my Lumia 640 XL OS to Windows 10 and I am loving it. Somehow I hate Android for many reason. That apart, I was eagerly waiting for this update/upgrade.
I read that it needed Update Adviser app on Lumia, hence I installed it. it checked for space and other prerequisites and started downloading update. I removed some apps, deleted maps and I was quickly ready for update.
Update went very smooth than expected. new experience is really very nice. Overall user interface, MS Apps and many more are new and exciting. I got many things which I wished for. Flashlight was one of them. Now I plan explore and write it here.