How do you change pressure hose going from heatercore to engin?

How do you change pressure hose going from heatercore to engin?
With the engine off and cool simply remove and replace the hose. There should be very little or no pressure in the system with the engine cool - you can remove the radiator cap to make sure.

Keep in mind alot of manufacturers use a quick release style fitting at the heater core. The release tool for these fittings is available at any auto parts store for under 10 bucks. I recommend the little white plastic ones, they always seemed to work better.

On a side note: make sure the line you're disconnecting is for the heater core. The A/C lines run to the evaporator core at the firewall also and they can look similar. Static pressure is usually 90-100 PSI and a large amount of freon discharged can kill you if you inhale it, or burn your skin badly on contact.