Samsung Galaxy S2 won't update to newer OS

[AC Question]

I received a used Samsung Galaxy S2 GT-I9100 today. I bought it from ebay, and now I am trying to update it to android 4-point-whatever-is-newest. However, the phone itself says that I already have the latest software (android 2.3.5), and Kies says that I already have the latest firmware. Now, I know this is not true, because the S2 is able to upgrade to at least Android 4.0!
I think the problem may have to do with the area it is from. I turned it on, and it was factory reset. Everything was in Chinese (and yes, I am certain that it was Chinese)!
This phone's carrier is unlocked. That may also be a factor in the problem.
I have searched everywhere, and all this "kernal" and "rooting" stuff has got me really confused. Any ideas?

 

[Golfdriver97]

I think almost all the S2 variants are up to 4.1.2. You could try flashing a Stock ROM in Odin on a PC with the most up to date one. Otherwise, you could try to root and flash something like CM 10.2 which is Android 4.3