Consider upgrading your phone. The Galaxy S3 with KitKat will still be very slow by modern standards, and most apps no longer support Android 4.4. Disclaimer: Flashing custom software can void warranties (already expired) and potentially brick your phone. Proceed at your own risk.
The Samsung Galaxy S3 (GT-I9300) was originally released with Android 4.0 Ice Cream Sandwich and officially received Android 4.3 Jelly Bean. Officially, Samsung never released Android 4.4 KitKat for the international GT-I9300 model. samsung galaxy s3 software update 4.4 free download
If you want KitKat on your S3 today, you must install a custom ROM like CyanogenMod 11. It is free, but you need patience and a computer. Consider upgrading your phone
