If the screws are in normal condition, you should be able to loosen them with only a wrench. But they can be tight sometimes due to constant exposure to water and air. In those cases, you can try a few methods. First, clean the rust off the screws using a wire brush and some rust dissolving oils. Next, lightly tap the casing surrounding the screws. This can help remove some of the corrosion. If you cannot loosen the screws even after this, try a power tool like an air hammer as a last resort. But don’t go overboard as you can crack the aluminium casing.