The starter of a Nissan Altima is located at the top front of the transmission. The air filter and inlet duct to the throttle body have to be removed in order to access the starter.
The starter is attached to the transmission with two 17 millimeter bolts. In order to remove the starter, these bolts have to be loosened with a socket wrench. Once the bolts are loose, the starter can be slid out of the car. Remove the wires that attach the starter to the battery before removing the starter. Always remember to disconnect the battery before working on the electrical system of a car.