How do you replace a garbage grinder?

Depends on what's wrong with it? If when the 'On' switch is thrown you can hear a humming sound it may be jammed with some hard debris. The debris can often prevent the motor from turning. Look under the sink at the bottom of the disposal unit at the very center of the bottom of the motor. You will find a hexagonal shape indent designed for a hexagonal wrench. The wrench was supplied by the installer and left for you in case of need and may be somewhere in the under sink cabinetry. Or not. (You didn't throw it out did you?) With the power switch turned off, fix the wrench into the indent and attempt to move the grinding unit. Turn the wrench to the right and to the left to try and dislodge the debris. If the debris is dislodged you can then try the power switch.

If still the unit fails to operate then look again at the bottom of the disposal unit for a small reset switch and press the switch in. Again, try the power switch. If still not working you may guess the unit is kablooey and needs to be replaced. BUT WAIT; first look at your main circuit breaker to ensure all is ok.

The remainder of the replacement work required can be done by the average Joe if you carefully follow the installation instructions contained within the box holding a new unit. I'll suggest you should buy a replacement unit with at least equal horsepower rating or more horsepower. Cost wise get more and never less.

Installation will take you a bit of time and you will need a set of plumber's wrenches, a screw driver, Teflon plumber's tape; don't forget patience with the instructions and a cold beer. Do it slowly, follow the instructions to avoid water leaks. Yes, you can do this and be a hero.