In order to update manual to CoD4x 20.6 Windows client make the next steps

Download required files from here cod4x.ovh (windows client 20.6)

Copy all the files, everything inside "cod4-client-manualinstall_20.6"

paste them to your cod4 directory

Locate after you pasted it file called install.cmd, doubble click it and press any key after the window pops up

Start your game it should be now on 20.6 version