Go to:
http://web.canon.jp/imaging/eos1dm2/eos1dmk2_firmware-e.html
Download Instructions:
http://web.canon.jp/imaging/dc/PDF/eos1dmk2_fib-e.pdf
Input Serial of Body, then download: eos1dmk2126.exe
Doubleclick this file in Windows, it is a self extracting zip file.
You will end up with: 1D200126.BIN in the same folder.
Format a memory card, 8Mb to 2Gb, CF or SD, copy 1D200126.BIN to the ROOT.
Insert the card into the body, turn on the power.
Menu>Section 2, Select>Firmware>OK Updating Commences>OK.
PDF File explains it in greater detail.
I have assumed you are running Windows, I know nothing about Macs.
By the way, if you want to know how many clicks your shutter has done?
I recommend:
http://www.nknu.edu.tw/~technology/exif/KUSO_EXIF_Viewer_EN.zip
Look for the Header: Standard Information & Field: ShutterCount.
FYI: I'm already on Firmware 1.26 and a ShutterCount of 117031!