Check Fus is a great way to quickly check, without even downloading, the latest official firmware for your region. This is very helpful for knowing what you got shipped, and what version you should download, and flash to get back to stock, in order to send it in for warranty.
This will only DOWNLOAD the ROM, and NOT flash it. To flash it, you will have to use ODIN, and the appropriate way of flashing it (for your phone).
-Here is a guide on returning back to stock on the Samsung Galaxy S GT-I9000
-Here is a guide on returning back to stock on the Samsung Galaxy S III GT-I9300
Here is a full guide on how to flash back to stock!
In this video I cover:
-JPU – Full ODIN flash
-JPY – PDA Only flash via ODIN – to make it able to connect to Kies
-JVO (latest as of 17/06/11) gingerbread ROM via Kies
Remember to wipe the INT SD and Factory reset before you send it in!
You have to register and once you download the files you will be prompted to enter a password. It is mentioned on http://www.samfirmware.com/ however here are the 2 variations of passwords you will need to extract the file to your PC:
## Password is = samfirmware.com
# Password is = samsung-firmwares.com