Push the hammer down toward the bottom out of of the frame, you will see the pawl (hand) is retained by a small screw. Remove the screw, pull the pawl down and out, then the hammer will come out the top.
Replace by inserting the hammer from the top, downward as far as it will go. Place the pawl up into its slot from the bottom, align the screw holes and replace the screw, pull the hammer upwards to align the pivot screw in the frame.