Here is the exercise to help you update Galaxy S3 to
Android 4.4 Kitkat based CM 11 informal form. The authority CM is not available in the market yet, so how about we do with the informal form for the present which has been created by a XDA dev. This post is customized to Samsung Galaxy S3 model I9300.
You must need the following things already before upgrading your system
> You must have a back up of your data before trying to upgrade your system
> Download and Install USB Drivers for Samsung Galaxy S3
> Enable USB Debugging Option
You can enable it by pressing MENU > SETTINGS > APPLICATIONS > DEVELOPER OPTIONS
> Make Sure that your phone is fully charged or better to be plugged in.
> Your Phone Must be Rooted
You can get the instructions to root your phone
here
How to Upgrade Your Android
Connect your Samsung Galaxy S3 i9300 to your machine utilizing USB link and copy the downloaded files to the SD card of your Galaxy gadget.
Disconnect the USB Connection and turn off your mobile phone.
Boot in to custom recovery mode: press and hold Volume Up, Down and the Power buttons simultaneously until Samsung logo shows up. Leave the keys for half a second and hold again. Use Volume keys to scroll and Power button to select - alternatively you can use Quick Boot app to reboot your Android device in recovery mode.
Now perform Nandroid backup of your current ROM: select Backup and Restore > Backup. Go back to main recovery menu after backup process is done - very important step to restore back to current OS.
Now Wipe Your Data, it may take few time
Select install zip from sdcard followed by choose zip from sdcard. Locate the Android 4.4 Kit Kat based CM 11 custom ROM file copied earlier and select it. Tap on yes to confirm installation and wait for the ROM installation to be completed.
Once installation process is finished successfully, repeat the above step but choose GApps file instead to install the Google apps package on your SGS3 i9300.
Return to main recovery menu now and select reboot system now to 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.
If you want to restore your Mobile phone in the previous condition then return to previous OS on your SGS3, boot into recovery, select backup and restore then restore your previous ROM by selecting it from the list.