1. Download the firmware
2. unzip it to extract the instructions and the .fir file.
3. Put the .fir file in the root of the card, don't put it in a folder.
4. Put the card in the camera, turn it on. (Make Sure your battery has a full charge)
5. Put the camera in the "P" mode, and go to the screen that shows the current firmware version. Select and click on it
6. The Firmware update screen will appear, showing the new file 1.2.1
7. Click OK, and OK on the confirmation screen, and OK to start the update.
8. Verify that the new firmware is installed.
I'd never use the Canon Utilities tether to update firmware, its best to keep it as simple as possible.