To determine the correct drill bit size for a screw, you'll need to consider the diameter of the screw and the material you're working with. Start by identifying the size of the screw, which is usually indicated on the packaging or the screw head itself. Select a drill bit that matches the diameter of the screw's shank. If you're unsure, it's better to choose a slightly smaller bit and test it on a scrap piece of the same material. 

To change a drill bit, start by ensuring the drill is turned off and unplugged for safety. If it's a cordless drill, remove the battery. Locate the chuck, which is the part of the drill that holds the bit. Many drills have a keyless chuck, which can be operated by hand. If you have a keyed chuck, use the chuck key to loosen and remove the old bit by turning it counterclockwise. Insert the new bit into the chuck and tighten it by turning it clockwise. Ensure it's secure and doesn't wobble.