Galaxy S2 has received the update to JellyBean 4.1.x this month (March 2013). Until the update is propagated through all regions of the world, you can't do nothing other than waiting. You will also need Samsung Kies in order to update it. Simply plug the phone to the computer and Kies will tell you if there is a new firmware update for your phone. If there is one available, make sure your phone is charged to 85% or above.