For a detailed instruction:
Go to
https://cod4x.ovh/ scroll down to "Download" and download version 21.1 for Windows client. It will download a .rar file, unzip it on your desktop and you will have a folder called: "cod4-client-manualinstall_21.0".
Go to
"C: \ Users \ YOURNAME \ AppData \ Local \ CallofDuty4MW \ bin" and in this last folder copy the "cod4-client-manualinstall_21.0" folder, open the folder and double click on the "install" file. A DOS window will open that will show the update and at the end of which it will ask you to press any key to close it. The upgrade is ready. Enter the game as you usually do and ENJOY on FNRP

NB: When you look for
C:\Users\YOURNAME\AppData\Local\CallofDuty4MW\bin and you don’t find the folder:
AppData go on folder “Local” select tools -> Select Folder Options, then select the View tab. Under Advanced settings, select Show hidden files, folders, and drives, and then select OK.
After done all upgrades of game remember to make hidden again folders
For this post i used part of
PUMA's instructions and also made extensive use of useful information that
ElementX gave me time ago for past upgrades.
Many thanks
