- Download the Root Box Based Android 4.1.2 Jellybean Custom ROM and Google APPS Package from here to your computer. After the download completes, connect your phone to the computer with an USB cable and copy the two downloaded zipfiles to the root folder of the phone's SD card.
- Disconnect the phone from computer. Then, power off the Galaxy S3 device and boot into the recovery mode. To do so, press and hold the Volume Up and Home key buttons and keep tapping the Power button until the device boots into recovery mode.
- In recovery mode, choose Wipe data/factory reset and Wipe Cache partition options. If thedevice accidentally boots into normal mode, then you need to retry the process by repeating the previous step. You can also enter the device in recovery mode using the ROM Manager app.
- Choose install zip from sd card and click choose zip from sd card. Now, browse and select the Root Box Jellybean 4.1.2 Custom ROM and Google Apps Package which you copied to your phone. It will take about 4 to 5 minutes for installing the update.
- After the ROM is installed, hit Go Back and tap Reboot System now to reboot the phone in the normal mode. The first boot is more time consuming than the normal mode, as it has to finalize all the apps. So, leave it alone.
Root Box ROM Based Android 4.1.2 Jelly Bean update is now successfully installed on your Galaxy S3 GT I9300. Navigate to Settings > About phone to verify the firmware version installed.
Source From: ibtimes.co.uk