Hi
No special tool is needed, you just have to twist that round-shaped retainer ring (anticlockwise IIRC)
For the first time it will be real hard (pressed into that position for years).
Take a large flat screwdriver or a small metal pipe put it against one of the protruding parts of the ring rim (keep it as horizontal as you can) and use your hammer carefully.
Be careful and patient, move it only a bit at a time going around and around.
Took me about 10-15 minutes for the first time.
Relieve the fuel pressure well, even a couple of drops inside will smell for long! Also, if you're at it, you might want to change the small L shaped filter at the bottom of the pump.
Tiv
