When I did the intake manifold work on our 380SLC I also replaced the seals, holders and O rings, and illustrated
the job here. Some of these pix may help - similar on your engine.
With abundant respect to Mike690003, I would not use a knife to remove the old seals as you risk scoring the injector body and creating a leak. Your choice, however. Slide on the new ones after oiling the injector shaft, like the factory manual says.
Also, I would loosen and remove the fuel lines (you may have to completely take them off if too much bending is involved - meaning undoing them carefully at the fuel distributor, using a brake line wrench if possible - see my pix) before undoing the 5mm Allen bolt retaining the injector retaining plate/spring - less stress on the line that way.
The injector line is 14mm and the fuel distributor line 12mm on the M116. Not sure of your sizes.