Lawnmower batteries are located in one of two places: under the hood or under the lawnmower seat. To find it, turn off the mower and remove the keys. Look in both of these places and release the cover latch so you can access the battery. If you find any strap or bar holding the battery in place, remove it. When removing the battery, make sure you remove the black or ground wire. If you find that the connectors are corroded, use a stiff wire brush to clean them. If the battery itself is highly corroded, use baking soda and water paste and apply it to the terminals. Leave for a few minutes before using a brush and water to wash away corrosion. Wipe off any remaining water with a soft cloth. Be careful not to let mixture or water get into any battery cells and rinse thoroughly before removing. Because baking soda neutralizes battery acid and will neutralize the battery if it gets inside the cells. But if the connectors remain secure, use a flathead screwdriver to slowly pry the connector away from the terminal. Make sure you don't unplug the cable as you may disconnect the connected terminal. When the negative cable is removed, the red cable is removed. Finally, when you have removed the battery from the holder, clean it of corrosion and dirt and dry the holder. Insert the new battery into the holder and attach the strap or strap. When reconnecting the cables to the battery terminals, be sure to start with the positive cable, which is red. Connect it to the positive terminal and tighten the bolt to secure it. Make sure you connect the negative cable, which is also black, to the correct connector. After that, put the cover back on and test it to make sure everything is working.