Clear a space under the sink to work. Remove any chemicals and hazards to avoid accidents and injuries. There may be some water spilled, place towels under the disposer to soak up the water.
Disconnect the drain trap from the disposer's discharge tube. It should be fastened with a locking nut. You may be able to loosen the nut with your hands, otherwise use a pair of adjustable pliers. Unscrew the nut all of the way.
If you have a dishwasher drain connected to the disposer, remove that connection now. There should be a hose clamp securing the hose to the disposer. Loosen the clamp and pull the hose off of the dishwasher inlet.
This is a view of the basic plumbing layout:
The disposer is fastened to a mounting assembly on the sink and is secured with a mounting collar. Before removing the old disposer, stack enough newspapers under it to hold it up. When you disconnect the mounting collar, the newspapers will support the disposer and prevent it from dropping. Do not lay under the disposer as it is heavy and could cause injury if it were to fall on you.
Insert the supplied wrench or a screwdriver through the mounting lug to turn the collar. Instead of a mounting lug, some models may use a tab like a wing nut. Support the disposer and loosen the collar. The mounting collar should turn clockwise (to the left) about 1/4 turn. If you cannot loosen the collar, use a mallet to gently tap the lug to break the seal.