Galaxy S3 I9300 users can now enjoy the latest Jelly Bean software with due release of Android 4.2.1 based CyanogenMod 10.1 (CM10.1) ROM.
Download CM10.1 Android 4.2.1 Jelly Bean ROM for Galaxy S3 I9300
How to Install Android 4.2.1 Jelly Bean on Galaxy S3 I9300 with CyanogenMod 10.1
Please have a backup of all of your files......
1. Connect your phone to the computer with the original USB cable.
2. Copy downloaded ROM and Google Apps zip files to the root folder of the phone's SD card. Important: Do not extract any files.
3. Disconnect USB cable from the computer and then switch off your phone.
4. Boot the phone into recovery mode. To do so, press and hold Volume Up, Home and Power buttons together until the screen flashes. Then release all of them.[use volume buttons to scroll and Power button to select].
5. Take a Nandroid backup of your current ROM that you can restore later if something goes wrong with the new ROM. To do a backup, choose Backup and Restore and then hit Backup again. Return to main recovery menu once backup is done.
6. Next you need to do a full-data wipe. To do so, click wipe data/factory reset, then choose Yes on next screen to confirm. Wait for a few minutes until the data wipe process completes.
7. Choose install zip from sdcard and then hit choose zip from sdcard. Now, locate the CM10.1 ROM zip file that you copied to your phone earlier in Step 2 and select it by tapping Power button. Confirm the installation by clicking Yeson next screen and the ROM installation will begin.
8. After the ROM is installed, repeat step 7 and choose the GApps zip fileinstead of ROM zip to install the Google apps package.
9. Once Google Apps is installed, return to the main recovery menu. Now, choose Reboot System Now to reboot the phone into CM10.1 based Android 4.2.1 Jelly Bean ROM. The first boot may take about 5 minutes to complete, so leave it alone.
10. Returning to Previous ROM: To restore your previous ROM, boot into recovery, choose backup and restore then restore your previous ROM by selecting it from the list.
CM10.1 custom ROM based Android 4.2.1 Jelly Bean ROM is now installed and running on your Samsung Galaxy S3 I9300
*Note-- We are not responsible to any damage done to the phone while doing the above mentioned process.
*try at your own risk*