How do I repair a magnetic contact?

My security system has magnets embedded in the tops of the doors that make contact with a sensor in the door frames. On one door the magnet has become too deeply embedded so it no longer makes contact. How can I remove it and replace it with a new magnet or bring it back into alignment so it doesn't sink into the top of the door? This is a hollow core door between two parts of the house. I don't know the make or model of the system as it was in the house when I bought it, but the house is 40 years old.