Canon EOS-1D X Mark II Update Manual page 7

(5) Update the firmware.
1. The screen will appear on the camera's LCD monitor.
2. If you press the <SET> button, a confirmation screen will appear, so check the firmware update shown, rotate the
Quick Control Dial to select [OK], and press the <SET> button again to start the firmware update.
3. The message will appear during the update.
* The firmware update takes approximately six minutes.
4. When the update is completed, the message will appear on the camera's LCD monitor.
5. Complete the firmware update by pressing the <SET> button.
The firmware update is now completed.
When the firmware update operations are finished, turn the camera <OFF> and remove battery from the camera for
at least two seconds. This will cause the new firmware to take effect after the battery has been reloaded and the
camera is turned on.
Format the CF card (or CFast card) before using it again.