How to change a starter in a 1990 cadillac sedan deville?

First, I would determine it is bad. I have a 1990 Cadillac Sedan Deville, and sometimes it doesn't turn over. But, if I toggle the switch a few times or up to 5 times, it starts to turn over and the car starts.

I have the same problem with a 1997 Cadillac Deville.

If you are set on changing it, here is a picture and instructions on how to test:
Starter TESTING Voltage Drop Test Fig. 1: Schematic of a voltage drop test
NOTE: The battery must be in good condition and fully charged prior to performing this test.

1. Disable the ignition system by unplugging the coil pack. Verify that the vehicle will not start.
2. Connect a voltmeter between the positive terminal of the battery and the starter B+ circuit.
3. Turn the ignition key to the START position and note the voltage on the meter.
4. If voltage reads 0.5 volts or more, there is high resistance in the starter cables or the cable ground, repair as necessary. If the voltage reading is ok proceed to the next step.
5. Connect a voltmeter between the positive terminal of the battery and the starter M circuit.
6. Turn the ignition key to the START position and note the voltage on the meter.
7. If voltage reads 0.5 volts or more, there is high resistance in the starter. Repair or replace the starter as necessary. NOTE: Many automotive parts stores have starter bench testers available for use by customers. A starter bench test is the most definitive way to determine the condition of your starter.
   

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Remember to disconnect the battery and remove the wires from the starter at the hex nut.

Then, remove the 3 or so bolts that hold the starter on using an extension and ratchet if necessary.

Hope this helps.