First unplug the disposal from the electrical outlet. Do not just shut the switch to the off position.
You will need to drain the water sitting in the trap so place a container under the trap to catch the water.
Loosen the collar on the disposal ( use a small hammer and screwdriver)
Remove the old gasket on the top of the disposal and replace with the new gasket.
Place a little plumber's silicon on the gasket and reinstall the disposal. ( it might be a little difficult due to the new gasket)
Check for leaks and then plug the disposal back into the outlet