How do I remove this tap valve?
The round piece that is around the valve stem appears to be threaded into the outer body of the valve assembly. The internal hex shaped hole is used to accept a tool that fits over the valve stem and engages the inside shape to allow that round piece to be screwed out. 



It is unlikely that a standard tool exists to extract the valve assembly and it may be necessary to contact the manufacturer to check on the availability of a specialized tool.