If its like my 95 E420 I believe there are 2 plastic screws on each black cover. Just unscrew and lift off cover. You might want to invest in a ignition wire puller. Its a plier like tool with plastic protective ends that assist in pulling off the wires from the spark plugs without damaging the connectors. Then use spark plug socket with protective foam or rubber insert to unscrew plugs. You will probably need extension on socket to assist reaching the plugs. My plugs were Bosch, purchased from FASTLANE and I did check the gap but they were already gapped correctly to my car's plug specs
Carefully screw in new plugs to avoid cross threading( you should be able to tell by feel if they are threading properly. I usually start them by hand turning then go to rachet handle to finish tightening. Probably should have torque wrench too, to tighten to proper torque spec. ( I have done this but most of the time I do not...I always seem to have troble with the torque wrench...feels like I have to tighten too much to get wrench to "click"and I am always afraid of over tightening.
I believe some suggest using an anti seize on the plugs...I never have but yes, sometimes getting the old plugs out takes some effort with the socket wrench.
At same time I changed plugs I did new distributor caps and rotors but I was at my 60K service interval.
I'm sure others better qualified than me will have other suggestions and tips. Good Luck
95 E420 ( 76K) SOLD