Here is the tutorial to help you update Galaxy S3 toAndroid 4.4 KitKat based CM 11 unofficial build. The official CM is not released yet, so let's do with the unofficial build for now which has been developed by an XDA dev. This post is tailored to Samsung Galaxy S3 model I9300.

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Prerequisitesbefore youupdate Galaxy S3 to Android 4.4 KitKatbased CM 11 custom firmware:

Create a backup of all important data in your Galaxy device.Download and install USB drivers forSamsung Galaxy S3in your computer.Enable USB debugging option by pressingMenu>Settings>Applications. Navigate and tap onDeveloper Optionsto ensure thatUSB Debuggingis enabled.Make sure that recovery tool like ClockworkMod recovery is installed.Ensure that your Samsung Galaxy S3 carries more than 80% of charge.

Guide toupdate Galaxy S3 to Android 4.4 KitKatbased CM 11 custom firmware:

Files required:download the unofficial CM 11build based on Android 4.4 KitKat andGoogle Apps. Save both files on PC.

Connect your SamsungGalaxy S3 i9300to your computer using USB cable and copy the downloaded files to the SD card of your Galaxy device.Disconnect the connection with PC and turn your GalaxyS3 off.Boot in to custom recovery mode: press and holdVolume Up, Downand thePowerbuttons simultaneously until Samsung logo shows up. Leave the keys for half a second and hold again. UseVolumekeys to scroll andPowerbutton to select -alternatively you can useQuick Boot appto reboot your Android device in recovery mode.Now perform Nandroid backup of your current ROM: selectBackup and Restore>Backup. Go back to main recovery menu after backup process is done -very important step to restore back to current OS.Data wiping tasks: selectwipe data/factory reset>Yes. It will take a few minutes; once done, selectwipe cache partition.Next tap onAdvance>wipe Dalvik Cacheto wipe Dalvik cache data too.Selectinstall zip from sdcardfollowed bychoosezip from sdcard. Locate the Android 4.4 Kit Kat based CM 11 custom ROMfile copied earlier and select it. Tap onyesto confirm installation and wait for the ROM installation to be completed.Once installation process is finished successfully, repeat the above step but chooseGAppsfile instead to install the Google apps package on your SGS3 i9300.Return to main recovery menu now and selectreboot system nowto boot up into the latest unofficial CM 11 build for Samsung Galaxy S3. It will take a bit of time during the first boot, so kindly be patient.

How to restore:If you want to return to previous OS on your SGS3, boot intorecovery, selectbackup and restorethen restore your previous ROM by selecting it from the list.