Turn off the power running to the unit from your breaker box.
Remove the screws holding the service panel to the unit casing, using a screwdriver.
Look for the drip pan on the bottom of the evaporator coil. The evaporator coil is the only metallic coil inside the unit. The drip pan is below the coil.
Pull the drip pan out of the housing. Remove the bolts or screws holding the drip pan under the unit, using a wrench or screwdriver.
Insert your new drip pan under the coil. Secure the drip pan in place.
Replace the service panel. Turn the unit back on at the breaker.