The clock face has 4 pins attached to the back which fit in holes in the front plate. At the rear of the front plate, those pins have transverse holes into which tiny tapered pins fit in order to secure them.

Remove each of the tapered pins in turn by gripping the thicker end with a small pair of pliers and pulling. These pins are easily lost - put them somewhere safe.

If the pins don't come out easily, make sure you're holding the clock mechanism firmly so as not to jerk it violently if a pin comes out suddenly.

You can now lift off the clock face and put it aside.

Do not attempt to remove the tapered pins by pushing on the thin ends - they will most likely only bend.