Remove the power cable from the wall outlet in your kitchen. Make sure that there is no power going to the microwave. Turn the microwave around so you have access to the back panel.
Remove the screws holding the back panel onto the unit casing with a screwdriver. Pull the back panel off of the unit to expose the components inside. The back panel should come off with ease.
Use the screwdriver to bridge the gap between the terminals on the capacitor. The capacitor should have 3 prongs coming off of it. Put the metal part of the screwdriver across the prongs until you hear a hissing noise. This means the capacitor has been discharged.
Locate the magnetron on the inside of the microwave. This is a cylindrical object in the middle of the unit casing. Use your fingers to pull the old magnetron out of the unit housing. Use a pair of pliers to pull the first wire off of the magnetron. Connect the wire to the wire terminal on your new magnetron. Do this until you have all of the wires off of your old magnetron and onto your new magnetron.
Place your new magnetron into the housing of your microwave. Replace the back panel and the screws securing it to the unit casing. Plug the microwave back into the wall outlet and test it out.