First you have to determine the source of the leak. Occassionally you will get a leak from the gasket where the drain connects to the unit and the water will seep down the outside of the unit in a very light film almost unnoticable, then form droplets and drip off the bottom of the unit. In that case, simply cleaning the seat and replacing the gasket will solve the problem.
If the water is leaking out of the electrical access cover or the weep hole in the bottom of the motor housing it would require dissassembing the unit and replacing the motor shaft seal.
I have known some repair techs who have had limited success in rebuilding residential garbage disposal units but in my experience locating the necessary parts is nearly impossible and even if one has access to the parts, when compared to the initial cost of the unit, the cost of parts and labor to restore a fractional horsepower residential disposal is simply not cost effective.