(To be 100% pedantic: The physical button is "Fn2". "Multifunction" is a software function that can be assigned to (some of the) physical buttons, and by default it's assigned to Fn2.)

Hold the button down and rotate a dial one step - either of the two dials, in either direction. Now you're selecting which of the 4 functions the button will be quick-accessing. Continue rotating the dial left or right, and when you've picked the function you want, release the button. From this point on, every press of the button activates that function. When you want to change it again, hold the button and rotate a dial anew. It may be a bit confusing at first, but IMHE it's easy to get used to, especially as your fingers gradually learn exactly how many dial steps there are between the functions.

At this point it would be hugely useful if you could also configure the exact list of functions you want covered by Multifunction - e.g. you might reasonably want quick access to ISO there instead of the aspect ratio - but this isn't possible; it's either that exact set of functions or nothing. If we're very lucky, some future firmware update might make that configurable too.