1. First of all, download'n'extract Odin.
2. Afterwards, download stock firmware. Extract it to Odin folder.
3. Pick up an USB cable and use it to connect your phone to your PC.
4. Turn the phone on in Download mode, then open Odin in Administrator mode.
5. Make sure that you have Samsung's recent drivers installed. If you do, an ID:COM field will be highlited and software will be described as Added.
6. Select the Auto Reboot and Factory Reset Time options.
7. Now upload these images: BL, AP, CP, CSC.
8. Press Start. Make sure not to disconnect phone from the PC. Patiently wait for the device to update.
9. Voila! After the phone is updated, you can safely unplug it. Done.