How do I fix the heater to blow warm air?

It's possible the coolant thermostat has failed, that would be a simple repair. If it's not simply a bad theremostat allowing the engine to run cold, there could be much more to it. The climate controls in a Cadillac are very complicated, there could be several factors involved including electrical or electronic. I wouldn't be able to tell you anything from where I sit. Get a Caddy specialist to look at it. Here's a directory link for you: http://repairpal.com/cadillac-repair-in-vista-california