You only need to know which game version you have bought and which update did you install previously. If not sure, reinstall your game and then apply all updates in right order depending on the original game version you have (you can determine your version number from the main menu of ArmA or from Windows desktop):
What version do you have?
Boxed ARMA: GOLD edition or ARMA Online Download (version 1.08)
* Download and install International 1.14 update only
US version ArmA: Combat Operations by Atari, box and download (Version 1.06)
* Download and install US 1.08 update
* When updated to version 1.08, Download and install International 1.14 update
European boxed DVD ArmA: Armed Assault by 505 Games (Version 1.04)
* Download and install Euro 1.05 update
* When updated to version 1.05, download and install International 1.08 update
* When updated to version 1.08, Download and install International 1.14 update
German, Russian, Czech or Polish Boxed DVD ArmA: Armed Assault or online download v. 1.0, 1.01, 1.02 and 1.04
* Download and install International 1.05 update
* When updated to version 1.05, download and install International 1.08 update
* When updated to version 1.08, Download and install International 1.14 update
Biki Update Guide