How do I remove Cyanogenmod 10.2.1-d2usc off my Galaxy s3 ?

[AC Question]

I recently bought a Samsung Galaxy s3 off of someone and they have Cyanogenmod on it instead of what it came with. I would like to know how to get it back to the way it came without screwing up my phone.

 

[Rukbat]

You install the ROM you want, stock or some other custom ROM. See [Samsung] How to flash Stock ROM via ODIN. (Most custom ROMs install the same way, or there will be installation instructions.) Flashing a ROM can be frightening the first time, but it's really a boring experience.

If you just remove the existing ROM without replacing it with something, the phone stops working.