Installation:
Copy it to the game folder.
BACKUP your original xrgame.dll somewhere. In the case of problems just copy it back.
Run fov_switch.cmd
Use the mouse wheel to scroll through all available variants.
In the opened window enter the number from 1 till 52 depending on your game version and chosen FOV.

I do not recommend to create a server/clients in Shadow of Chernobyl by the Internet, using changed xrgame.dll.
It will not be possible to connect. Or it will be necessary to agree in advance that on all computers in a network would be the same xrgame.dll.
But in Clear Sky it's possible to connect by internet with the machines which have not this patch.

Do not forget that either in Shadow of Chernobyl or in singleplayer of Clear Sky the external camera modes by F1-F3 are supported.
Plus and minus buttons on the right of keyboard are used to zoom.
Also you may use F4 in mp to switch into free-fly mode, when you are offline.
To enable it you should use the console and enter four commands:

bind cam_1 kF1
bind cam_2 kF2
bind cam_3 kF3
bind cam_4 kF4

To enable the external cam in singleplayer mode of Call of Pripyat (by F6-F8 buttons) just enter these commands:

bind cam_1 kF6
bind cam_2 kF7
bind cam_3 kF8

Also in all games it's possible to use unofficial external mode by adding -psp into shortcut or into *.cmd file you start the game by.