I've done it several times now.
With a pair of needle-nose pliers, grasp the locking ring where the raised ridge meets the open slot, and pull it out away from the handle, and also toward the end. It should pop up over the pivot pin, and off the end.
The pivot pin can be difficult. It's basically a rivet, with a large round head on one end, and it's pounded at the the other to keep it in place. If there's enough of the end showing, you can try filing off the excess to make the next step easier.
To remove the pin, use a hammer and punch. (I use an old drill bit shank that happens to be the right size.) Drift the pin out through the handle and blade tang, and then remove the inner (non-rotating) ring around the end of the handle. The blade will just fall out of the handle slot.
That's it. Not many parts on an Opinel.
Reassemble in reverse order. Don't worry about hammering the end of the pivot pin/rivet; the outer locking ring will keep it from going anywhere. Getting the ring back on is harder than getting it off; I use the pliers as mentioned above, and also a large flat screwdriver to slightly open the ring up at the slot a bit. Pull the edge of th ering away from the handle to get it over the round end of the pivot pin, and it should just pop into place.
If the locking ring is too loose, remove it again, and squeeze it more tightly closed with a large pair of pliers, then carefully reinstall it, taking care to not open it up any more than necessary.
That should do it!
