Always disconnect the battery before removing the starter motor, which is often awkwardly placed low down on the engine. You may have to raise the car on axle stands and work from below.
With a pre-engaged starter, the heavy lead from the battery is connected to the solenoid mounted on the starter body. Take careful note of the other leads to the solenoid before you disconnect them.
Remove the two or three bolts holding the motor to the engine backplate.
Lift out the motor, watching for any spacer shims. Collect them and make a note of their order so that you can refit them correctly.
Reverse the removal procedure to refit the motor.
Testing a motor for short circuits
Use a test lamp fitted with crocodile clips to make these tests. You will have to remove the endplates from the motor. The field coils are insulated from motor casing by wrappings. A break in the wrappings can cause a short circuit.
If the field-coil test below indicates a break, and the break is only a minor one, an auto electrician may be able to repair it.
If the armature test below shows a short circuit, fit a new or exchange motor.
If there are little splashes of solder on the motor, or any other sign that it has overheated, fit a replacement motor.