A leaked version of Android 4.2.1 for the Galaxy S3 I9300 got out just yesterday, thanks to an Xda member, Saturn, and Sammobile. An official update to Android 4.2 for the Galaxy S3 is still to be released though, as this is still in a testing phase. We expect at least a month or even more before the international Galaxy S3 gets an official 4.2 update. Until then, this leaked version of Android 4.2.1 XXUFMB3 works just as fine as an official firmware and you can install it yourself using Odin.
Regarding the features of I9300XXUFMB3 Android 4.2.1 firmware, we can notice the widget lockscreen and daydream that we already know from the Nexus devices that have Android 4.2. We also know it has improved ripple effect on the lockscreen, the notification center is modified, improved voice command to control the phone. By now these are the features that stand up, but there’s probably more to expect from this firmware.
The firmware details are Android Version: 4.2.1 – JOP40D (Jelly Bean), PDA: I9300XXUFMB3, CSC: I9300OJKFMB3.
If you want to manually install the firmware, you are going to need to use Odin. Remember we cannot be responsible for any damage that may occur to your phone if you do decide to install this unofficial firmware. However if you follow the steps properly and if you have enough knowledge about what you are doing there shouldn’t be any problems.
You can download Android 4.2.1 XXUFMB3 from here - I9300XXUFMB3_I9300OJKFMB3_ILO.zip.
After you download the firmware files that can be flashed with Odin you can follow the steps in this tutorial and install the firmware manually : How to install Android 4.2.1 I9300XXUFMB3 on Galaxy S3 I9300.