Need help unrooting!

Hi, so today, I just traded my iphone 4s for a used Galaxy s3. Whenever i turn on the galaxy s3, it shows Cyanogen mod. I want to remove this. I've heard that this is rooting but I just want to go back to original factory reset device. I'm new to android so anyone that would help out?

I've tried a lot of tutorials out there and it still doesnt work. Please tell me what I need to do this.

Thanks,
Eric

Make sure you have the same model [Only registered and activated users can see links. Click Here To Register...]

ugh fail cm >>>>> touch wiz
but yeah that guide should work
make sure if you're planning to take it to an ATT store that you use the triangle away app to kill your flash counter before you unroot