I have the same tempering valve in my boat. I have ruined one due to lack of winterization. It contains a wax pellet and sliding valve, something like a thermostat. There is no way to get antifreeze into its in'ards, other than charging the system with pink antifreeze and heating it up enough to make the valve move, which I judged to be a bad idea.

Take the philips head screw out of the knob. You will then be able to remove the knob. Note that the knob has features underneath that engage the brass casting, limiting the amount you can turn it with a stop (safety thing). Make some notes or take some pictures as to the position of everything, then unscrew the stem using the knob and counting the turns. You will need to remove the knob a couple of times and reposition it over the stop to continue unscrewing it. I think about 2 or three turns it will remove completely. There is a spring in there too as I recall, the rest is a brass casting with open passages in it. Now drain everything and blow it out. I left the stem out until next year just to be sure it didn't freeze. When you reassemble, screw it in the same number of turns, put the knob on the same way on the spines, etc. You may want to adjust it when all is done to set the hot water limit to something less than scalding.

I am about to do the same thing here in the PNW.