Hi RahulDubey221992 !
Windows 10 Mobile Preview has been released for developers for users on many Lumia Devices. I personally wouldn’t recommend the Windows 10 Mobile Preview as it has got a lot of bugs but if you really want get it download the Windows Insider app and upgrade. Most of us are expecting Windows 10 Mobile OS to be released
by the end of next month .... Not a 100% confirmed**.For the supported list of Mobiles Follow this link**http://windows.microsoft.com/en-us/windows/preview-supported-phones
Hope this Helps!