You just pull out the two little silver pins. Put it in the middle and clamp a set of mole grips on the pins and move slightly back and forth as you pull the pins. Or send them to me and I will give you replacements which are already a bit tighter. If you are going to do it yourself wash with soapy water taking the orings out and cleaning the O'rings and the groove they sit in, and also the inside of the large plastic piece where the disk is.
Peter