Yes when changing a vehicles engine thermostat you need to drain down part of the coolant so that it's below the top radiator hose that connects to the thermostat housing.
Make sure the engine is cooled down before draining the coolant.
You usually only need to drain about 1 gallon of coolant in the radiator to get the coolant level down below the thermostat housing.
When you do remove the top radiator hose there might be a bit of coolant that comes out but there should not be a whole lot.
Then just make sure you refill the radiator with the coolant you took out or put some fresh coolant in.
You can usually attach a small hose to the radiator spigot and drain the coolant into a bucket or other container but you only need to drain a bit.
You should also remove the radiator cap to allow it to vent while draining the coolant.