First of all you need to access battery. For this, raise the hood of the car to see the battery clearly. See the cables and connectors on the battery coils, if they have too much corrosive material around them, clean it with a wire brush so that you can see the collectors and cables clearly. If you leave the battery open, the level of corrosive material will be excessive and will need detailed cleaning.
You will need a wrench to loosen clamps or connectors connecting the cables to the battery's coil. Carefully, loosen the clamps, it is likely that they have been eaten up by the corrosive material completely. Using wrench carefully also avoid a possibility of slipping the wrench off the corrosion.
Remove negative cable from coil of the battery. Removal of first negative cable is must to avoid any spark or related incident. An old battery frequently emits gas and corrosive material, which is explosive and can cause a risk of injury if the negative cable of the battery is not removed before the positive cable.
Removal of Cable from Anchor Point
After removal of negative cable, trace it to its anchor position, which most likely will be on top of engine. Also, it is possible that connector on the anchor side of the cable is also corroded and will need cleaning with a wire brush before removal. Carefully brush the connector and remove the cable.
Removal of Positive Cable
Remove the positive cable of the battery the way you have done the negative cable. Clean the clamp first with a wire brush and remove the cable from coil of the battery. Then trace the positive cable connection down on the anchor and clean it before removing it from that end. Your job of cable removal is thus done safely and securely.