want to downgrade my operating system on galaxy s3 GT-19300 i have updated my galaxy s3 GT-19300 to android kitkat 4.4.2 
now i want my earlier version back on my mobile 
so what is the procedure for that
From your question its clear that your phone is rooted. So if you want to go back to CM 10.2 from CM 11 , you need to download stable build from Cyanogen Portal for your device and flash it using ClockWorkMod recovery or any other compatible recovery. Similar to how you done with CM 11. You can backup App Data. But its recommending a clean install.