If we are installing directly UHD then why many environment are needed?
UHD is not needed to use LabVIEW. If you are using NI USRP hardware, all you need is the NI-USRP driver and LabVIEW. If you are using Ettus hardware, there may be other steps needed first to get the hardware recognized by your computer. What hardware are you using? I recommend looking up the corresponding getting started guide to get your equpiment up and running.
Please tell me anyone exact procedure for FM implementation by using GNU Radio and NI USRP-2920 .
Secondly, by using NI USRP-2920 and LabView.
There aren't very many people that use GNU Radio on these forums. Most people use LabVIEW.
If you are using LabVIEW communications, there are several examples and guided help tutorials on how to use Frequency modulation.
If you are using LabVIEW 2016, I recommend checking out the community page: https://decibel.ni.com/content/docs/DOC-40622
There are several examples there.