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.
Posting this as an answer so I can post a picture, but this is in addition to Michael Karas's answer.
There is a very common tool known as a "shower valve socket" that are sold individually and in sets at most stores that sell plumbing supplies. I would hope the makers of the faucet used a somewhat standard size, but the sockets come in several sizes, so finding a close fit should be possible.