We cannot be held liable of any damage done to your device while following the below mentioned procedure. Do at your own risk.
Install Android 6.0.1 Resurrection Remix Custom ROM on Galaxy S3 I9300
Take a complete backup of data,apps and settings as the procedure may wipe your data.
Make sure that the battery of device has a minimum charge of 60%.
Enable “USB debugging” on your device by going to Settings >> Developer options >> Enable USB debugging.
Connect your device with computer via USB cable.
Copy/paste the downloaded Android 6.0.1 ROM zip and Google Apps zip files to your SDcard root folder.
Switch off your device.
Enter Recovery. Hold power button and volume down button and enter recovery.
Select “wipe data/factory reset”, wipe cache partition” and in Advanced select “Wipe Dalvik Cache”.
Select “install zip from sdcard” and then select “choose zip from sdcard”. Navigate to “Android 6.0.1 ROM” file which you have downloaded and confirm to “install”. Wait till it gets installed.
Repeat the procedure to install Google apps by selecting “choose zip from sdcard” and then select “Gapps.zip” and install.
Select “reboot system now”.