I'm unsure of how to remove the head of this old TRV. It looks like part of it has been removed already. I haven't tried removing or unscrewing anything yet.
It best to open the valve fully before undoing the union below the actuator. This is a thermostatic valve and upon reaching temperature pressure is applied downward to the valve concealed in the 90° fitting. It will be easier if the radiator set point is set high, allowed to warm and then just before removal set to the lowest setting. You can often undo the union by hand by using this method.
Nut and direction