After making the necessary preparations mentioned in the article Things to Remember Before Unrooting and Flashing Samsung Galaxy S3, all you have to do is follow the guide below. But before that, we would like to reiterate that this method is definitely very dangerous because it has the tendency to cause errors or it may brick your Samsung Galaxy S3 if not done properly.
Unzip the ODIN or custom firmware that you have downloaded if they came in a compressed folder. You can simply use WinRAR or 7-Zip for them.
Shutdown your Galaxy S3 and connect it to your computer using its USB cable.
Launch ODIN by clicking its icon.
Set your smartphone into Download Mode. You can do this by holding the Power, Home and Volume Down buttons until the display shows a Warning message.
In ODIN, see to it that the Re-Partition box is not checked. Then, click the PDA button. From there, select the tar.md5 file that you have unzipped (it’s the one that came from Sammobile).
Now, all you have to do is hit the Start button, which is located below the small white rectangular boxes. After that, let the tool do its magic.
When the Pass message appears in the green rectangular box on the topmost portion of ODIN, it will trigger a reboot in your Galaxy S3. If somehow you get stuck in an infinite loop, just remove the battery of your unit and boot into Recovery Mode. You can get to Recovery Mode by pressing the Home, Power and Volume Up buttons altogether until the Samsung splash screen appears.
Press the Volume Down button until the Wipe Data/Factory Reset choice is highlighted. Next, press the Power button to select it. That’s it, you just unrooted and flashed your Galaxy S3!
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