How to replace starter on 1998 Isuzu Trooper?

you have to diconnect the exhaust system on the side that the starter motor is located on. To reassemble you have to replace the exhaust gasket with a new one. If you are replacing the starter, there is a GM starter that is a direct replacement for the Isuzu starter and costs quite a bit less.
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If you decide to unbolt the exhaust pipe, unplug the Heated Oxygen (HO2S) sensor connector-threaded on the pipe-and negative terminal from the battery first; then proceed to remove the starter

Park your car in a safe place with a level surface and open the hood. Disconnect the black, negative battery cable.

Raise the front of the vehicle using a floor jack and safely support it on 2 jack stands.

Remove the starter motor heat shield from underneath the vehicle (driver side) using a wrench or ratchet and socket.

Disconnect the battery cable from the starter solenoid using a wrench or ratchet and socket; then remove the second wire connection on the solenoid using a wrench or ratchet and socket.

Remove the two starter mounting bolts using a ratchet, socket, ratchet extension and universal joint to give yourself extra clearance if necessary.

Work the starter motor out of the engine compartment and away from the vehicle.

Set the new starter and solenoid assembly in place and install the two mounting bolts.

Connect the battery cable and wire to the back of the starter solenoid; install the heat shield and lower the vehicle.

If you do not want to disconnect the black, negative cable from the battery, make sure to wrap electrical tap around the terminal of the batter cable as soon as you disconnect it from the starter solenoid. This will prevent an accidental electrical short and possible damage to sensitive electrical components in your vehicle.