Use the menu and navigate to the custom functions menu (it's the "camera" icon tab along the top row of tabs in the menu -- toward the right).
Select "C.Fn I:Exposure"
Use the rear-dial to navigate to the 3rd option "ISO expansion", press the select button in the center of the rear-dial, the use the rear dial to highlight "1:On"
You can now exist the menu system.
The camera will now have enabled a new "Low" mode (ISO 50), as well as "H1" and "H2" modes (ISO 12,800 and 25,600).
Be warned... the H1 and H2 modes are normally disabled because you'll get a lot of noise at those high ISO levels.
Just keep in mind that using expansion ISO is no different than doing it in post - only you have a lot more control in post.
If you're shooting RAW, which you should be, simply moving up the exposure is the same thing your camera is doing in expansion ISO. But you can choose to only go up as much as needed for any given shot.
Some cameras have an extended range below their 'native' ISO. Which would be 100 for Canon and 200 for Nikon, BTW.
Extended ISO in either direction has penalties attached. There is great debate as to whether ISO 50 is better than ISO 100.
But penalties are severe and noticable if you extend into the higher ones. They are really intended for the folks that say, I need the shot and I don't care how it looks as the shot is more important.
There is great debate as to whether ISO 50 is better than ISO 100.
Where are these great debates? If you shoot in RAW, and understand what ISO 50 means in a Canon camera, there's nothing to debate. Of course it isn't better, because the RAW file is just an ISO 100 shot with Metadata that tells your processing software to reduce the exposure 1 stop.
One more time for those who missed it. If you shoot in RAW and put your camera in manual (let's say f/8, 1/100) and take a shot at ISO 50 and ISO 100, they will produce the same RAW file. The difference is that ISO 50 shot has an exposure compensation of -1 eV. So a RAW converter will pull it down 1 stop. No different than if you grabbed the slider and did it yourself.
I'm not saying ISO 50 doesn't have it's uses. If you shoot in auto then it will meter to -1 eV. Or if you don't shoot RAW. Or if you want to use a long shutter speed and don't feel like adjusting exposure in post. But there's nothing magical to expanded ISO, it's just having your post processor automatically do something you could do yourself.