Not sure if it will work on yours, but can give it a try....
Slide your hand under the armrest, head-on. Tilt the rear of the armrest up and you should have a screw visible on both sides of the track. Remove these screws and slide the armrest out of the way (up). You can now put the slides back on track.
Hope this helps, or at least gets you started
76 240D (W115) - 550K miles
78 300D (W123) - 200K+ miles