I have a Samsung Galaxy SII (GT-I9100). OS version-2.3.3. Whenever I connect it to Kies, it cannot find an available software upgrade. Is there any other way to upgrade my OS?
Sure, just be aware that doing so you'll lose the warranty! If you want to do it anyway, just go to sammobile.com and look for your phone (just type i9100 in the search box). In the result list, choose your firmware and download it. To flash it, follow these steps:
Your Samsung Galaxy S2 I9100 is now successfully updated. Go to Settings > About Phone to verify the firmware version installed.