You simply remove the lines from the engine to the heater, they'll have coolant
in them. Once the heater is re-installed and the hoses reconnected, you may have to burp the engine to abvoid over heatin ghte engine because of an air bubble in the those lines.
Each engine is different, this is how we do ours:
101 - How to Burp Your Engine (Reply #6) Engine overheating problem (with Instructions "How To Burp!")
Many engines just unscrew the petcock on the top of the thermostat and rev the engine to push out the air.