I would get a bucket, or catch can of some sort, and remove the lower radiator hose. That will allow quite a bit of the coolant to drain from both the radiator and the block. Reconnect the hose and fill with water and run the car until it heats up, then drain the system again. Rinse and repeat until you have most of the dex-cool out. Then refill the system with regular coolant and water mixed 50/50.