If the light on the dash is comming on that only means the front pads are worn. You have to look at the rears to decide.
To add to what I put up earler ( and others have already mentioned most)
Remove the reservoir cap and drape something over it so nothing falls in. I usually just rest the cap on top.
The calipers can stay on but do one pad at a time because there is a piston on each side and it makes it easier to push in that piston if the other pad is in place.
I used channel locks to squeeze in the old pad/piston before removing the pad.
Replace the sensors whenever you change the brakes. Sensors are cheap.
I have used a drift pin, a nail and a small phillips head screw driver to push out the pin, what ever works.

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1983 300SD
200000miles