Download a version, install with your CD-Key and overwrite with your normal .exe.
You may have problems with needing a CD in your disk drive. There are also ways around this, also on this site. Rather old however.
the easiest way around this is google arma2oa no-cd crack, jihad. Find one and download it. usually it is just a replacement .exe file that you use instead and it will run without a disc. I honestly don't know if you even need to have the game disc in with arma in the first place, as i have steam.
Also, if there is an issue with not being able to install without it being a "disc", you can mount it do a virtual drive with a program called poweriso, or something equivalent. I would google how to do this, as it isn't terribly difficult and there is tons of stuff on google about how to use those kinds of programs.