A few years ago I ended up just changing loads of these on a new build estate. The customers would move in and turn them off and the minute they did they leaked. I have a feeling the heads are pressed on so you would have to pry it apart with patience a hammer chisel and thin wide screwdriver. We wouldn't warranty trying to fix them so just recommended changing and the customers were happy with that.
They are ceramic discs inside so its unlikely there will be a gland to tighten, either the cartridge seal or more likely that the spline o ring has gone which means those valves are set for the bin anyway