Re: i want to know that whether window 10 update available in lumia 640 xl .
Official release of the Windows 10 mobile is not yet available, but it should be sometimes in mid December.
However, you could join the Insider Program and help shape the OS itself, that way you'll get to see how will the OS look like before others can.
Right now, with currrent build, things are better than ever. Many of us are using it as their daily driver and quite honeslty, i have never thought of going back to 8.1. I'm on the 640, so we have similar devices and specs, so you'll get approx. the same experience, which is great by the way.
As i said, if can't wait for the offical release, you can join the Insider program. If you decide so, we're here to help you.