How to Update Samsung Galaxy S3 to Android 4.3 JB CM 10.2

CyanogenMod team is working on releasing its official version of CM10 based on Android 4.3; but till they do it, why not indulge in some unofficial Android 4.3 beauty with the help of unofficial CM 10.2 test build. This comprehensive guide will help you flashing the unofficial CyanogenMod 10.2 test build based on Android 4.3 Jelly Bean for Samsung Galaxy S3.

Prerequisites to update Samsung Galaxy S3 to Android 4.3 JB CM 10.2 custom firmware:

Backup any important data from your Samsung Galaxy S3 as a complete wipe will be required during the flashing process.
Enable USB debugging option on your device: Settings > Development Options > USB debugging.
Your device must be rooted, unlocked and installed with a recovery tool like TWRP or CWM.
Your device must be charged to at least 60% of power.

Note: the tutorial is only focused on Samsung Galaxy S3 I9300, kindly do not try this tutorial on other Android or Samsung devices. This guide would be particularly helpful for those who want try the AOSP ROM to see the new features of Android 4.3 JB.

How to update Samsung Galaxy S3 to Android 4.3 JB custom ROM:

Required files: download the CM 10.2 unofficial ROM zip for SGS3 from here and save it on your computer. Do not extract the contents.

Connect Samsung Galaxy S3 I9300 to your computer using a USB cable and copy the zip files to your phone’s internal memory card.
Disconnect the USB cord and power off your Galaxy S3.
Boot it into recovery mode.
Perform a Nandroid backup of your current OS; important step to revert back to current state. Go to Backup > backup and restore on CWM.
Wipe data: from CWM recovery mode, select wipe data / factory reset and confirm the process. You can skip this step if you are on a CM 10.1 ROM.
Now, to flash the CM 10.2 unofficial custom ROM, select install zip from sdcard followed by choose zip from sdcard. Locate the CM 10.2 Android 4.3 zip and confirm the flashing process.
When done, follow the above step for GApps package installation.
Return back to main recovery menu and reboot your S3 I9300 into normal mode. First boot takes a good 5 minutes so kindly be patient.

Awesome! You have successfully flashed the unofficial CM 10.2 custom firmware based on Android 4.3 Jelly Bean on your Samsung Galaxy S3 I9300.