Bertrand,
A large well fitting flat blade is needed if you don't want to damage the cap in the handle knob.
The cap is made from aluminium and has an O-ring on it. It shouldn't be too tight unless salt has found its way in.
There is then a phillips head screw, inside at the bottom of the hole, that holds the knob to the handle.
You will need a snug fitting phillips driver for this as it has loctite on it. A firm hand will get this undone.
Hope this helps,
Ken