No heat!!......is this the thermostat? how easy is this to replace?

Is the engine temp normal? If so it is not the thermostat. I would try flushing it first, the core may be blocked or partially blocked. Next I would confirm the blend door is working properly. Flush it first and post the result. Blend door is lesson number 2.

Most likely the core is blocked, Mine does this every winter. Go get it flushed and see what happens.

I assume you mean no cabin heat, and the first thingI would check is coolant level. In most any vehicle, low coolant or air in the system will affect the heater core before anyting else (highest point). If coolant level is good and you're sure you've bled the air out, is sound advice. Be sure that the control **** is still connected to the air door. Since you didn't specify year and model, I can't tell you if the air door is vacuum, cable or electronic.

If you mean the engine never gets to OT then, yeah, most likely the T-stat is stuck open or has grenaded.

you can get the car up to operating temp then feel both hoses going to the heater core if they are both hot it's not clogged.