Changing a car battery safely involves wearing gloves and safety glasses, using the proper tools for the job, making sure the keys are not in the car's ignition, and disconnecting and connecting the terminals in the correct order. Batteries are filled with acid and can explode.
Car batteries are heavy, with each one having roughly 24 pounds of lead. That is in addition to the acid and other components. The acid is the main concern because if it comes into contact with eyes or skin, it can burn. Stray sparks can cause the acid to ignite, which is why taking the keys out of the ignition is important. In most modern cars the key completes the ignition circuit, even when the car isn't running.
The old battery should be cleaned before removal to check for cracks, which may allow the acid to leak. Placing duct tape over cracks provides some protection, but it's best not to touch damaged areas. Any tools used for the change should be kept on the ground or in a plastic tray on top of a cooled engine. Metal tools touching the metal on a car's engine can create sparks.
The negative terminal should be removed first, using a wrench or pliers in one hand to turn the nut and a pair of pliers in the other to hold the bolt head in place. After disconnecting both terminals, the plate that holds the battery in place may be removed. Lift the battery out, using the attached handle if it's in good shape, or by carefully lifting it by the sides. Don't allow it to tip, which may cause the acid to spill out.
Place the battery on the ground and away from the vehicle before installing the new battery. Clean the terminals using baking soda and water or a store-bought solution, then lift the new battery into place. Connect the positive and then the negative terminals, then secure the plate or battery support to keep it in place. Remove all equipment and tools from the engine area, and start the car.