Before we start: if you are replacing seals after a period of long use, now is a good time to do a descale and clean of your machine.
I have found descaling the machine can make the seals in pistons feel squeaky, I think it removes some (or all) of the lubricant, and it can even lead to a small drip leak from a group. As we are going to replace and lubricate the Seals anyway we should Do this Job Now....
Where I live the water is hard so I mixed three teaspoons of citric acid powder with 3 liters of water and completely filled the boiler. I let is sit for 20 minutes and then turned the machine on. As it came up to heat I blasted water out of the steam arm (when completely filled water comes out of the steam arm) into a milk jug. I then blasted steam through the boiling citric water in the jug to thouroughly blast out and clean the steam nozzle tip. After doing this I blasted water through both groups and the hot water arm also into a metal frothing jug. I ran most of the solution in the machine out like this.
I let the machine cool, filled with plain water and drained the boiler using a hose to syphon it out of the top water fill hole. I did this twice to completely rinse out any citric acid taste.
(For a machine with really heavy use you may want to use a commercial descaler or repeat the citric acid flush a few times.)
Ok -my machine is fresh and clean- just how I like it- the sight glass looks perfectly clear again. Time to switch over those seals.
First we need to remove the shower screen.
Do this with a flat head screw driver. On the side of the screen there is a small indentation. Brace the driver against the bayonet of the group and pry the filter off. Dont be afraid: it is clipped on and will pop off. Dont scrape around at the top edge of the plate: catch the driver on the ridge like so: