I changed mine on a 87 420, so it should be pretty similar.
I had to remove the air filter.
The wire set I bought comes with different length wires, so you want to compare them first. I then labeled them, so the long ones would go to the back.
I wrote down the order of the wires as I pulled them, as they are routed somewhat irregular (see picture)
The cap comes of after 3 screws and the rotor comes off by simply pulling it up.
I put dielectric grease on the metal connections and put the new parts in.
A spark plug wire puller helped to get into the hard to reach corners.
Hope this helps.