As discussed in answers and comments, channel pliers or locking pliers can be used to remove the screwed down cap that hold in the stem (the grooved top of which you show). As indicated, tape or a cloth may reduce the chance of damaging the finish. A wide, thick rubber band is also useful in gripping under pliers to avoid scratches.
Best Answer
It looks like to me that you need to tap the inner ring indicated with screwdriver and hammer.