I did this awhile back on my '91 300 SE, so I'll walk you through it.
WARNING, be careful with those springs!

The shocks hold the swing arm from going down too far; but there is a little tab on the swing arm that contacts the frame, and stops the swing arm from flopping down if a shock breaks or is removed. We'll use those as our safety net.

So,

1. Remove the top shock bolts (behind the rear seat).
2. Remove the lower shock bolts. notice the shocks are now loose, but will not come out the bottom, due to ground clearance.
3. Jack the car up, placing the jack UNDER THE SWING ARM, near the lower shock mounts, leave enough space for the shock to slip out. As you jack the car, the shock will slip out.
4. Now, place jack stands under the FRAME of the car (at the normal jacking points).
5. Slowly lower the jack from under the swing arm. As you do this the swingarm will come down, and will eventuallY stop when the little tab hits the frame.
NOTICE; WATCH THE BRAKE LINE. one of my little tabs allowed the swing arm to lower enough that the brake line stretched pretty tight. We don't want any stress there. Remove one end of the brake line if necessary.
6. Okay, we have the swing arm all the way down on the tabs, so that's as far as it will go.
7. Now, compress the spring, only as much as needed, and pull it out - this is the scary part.
8. compress the new spring a bit and put it in, positioning the upper rubber seat properly, and de-compress the spring.
Then slide the shocks in, and jack up the swing arm, position the shocks and bolt them in.

Be very careful with the spring compressor, tighten it only as much as needed to get the spring out, and don't get any hands in harm's way.

By the way, my springs were sagging a bit, but I just used a slightly thicker upper spacer, and leveled the car quite well without changing the springs.
the dealer sells these pads in 3 sizes.

DG