Because the mechanism of a grandfather clock is entirely exposed to the air, it has to be rebuilt about every ten years. That is why they are so cheap on the resale market. If you buy one costing several thousand dollars, it will not only be more beautiful but its value will go up with time instead of down.
To answer your question: There is a nut on the end of the pendulum to raise or lower the bob. If you lower the bob, it runs slower. If you raise it, it runs faster.