I didn't find any instructions on the web on how to do this, so here are some. It wasn't obvious to me at first, but in the end, there really is only one thing to do: remove the one visible pin near the clasp:
I recommend you take pictures, just like I did... Once the pin (well, pins if you adjust both sides) is/are removed, one can slide the outer link off the inner link:
Note that there are two sizes of outer and inner links; keep track in order to put them back correctly.
One has to allow the inner links to bend over in order to remove the outer links.
Here's a picture illustrating the different sized inner and outer links. Assembly is the reverse as they say. All in all, a clever system.