To replace the secondary drip pan in a central air conditioner, open the panel on the unit and locate the secondary pan on the bottom. Secondary drip pans are designed to be removed easily and should just slide out. Remove the secondary drip pan. Insert the new pan and close the panel.
There are two drip pans in most air conditioners: a primary pan welded underneath the coils and a secondary one at the bottom of the unit. The primary drip pan is permanently attached to the unit and cannot be replaced. It can, however, be repaired.
To repair the primary drip pan, remove the water from the pan with a wet vacuum and dry the pan with a towel. Let it air-dry thoroughly before continuing. Identify the area on the pan that is leaking. Use a fine-grain sandpaper to clean and rough up the area around the leak. Apply water sealant to both sides of the pan at the site of the leak. Allow the sealant to cure, following the directions on the product. Sand down any excess glue and test the seal by pouring some water into the pan. Always disconnect or shut off the power to an air-conditioning unit before opening it.