Radiator Hose Change
Drain the radiator from the petcock into a container using a funnel. Plastic milk jug, etc. Have several on hand.
Dont just let it run out on the ground as it is toxic to pets/animals.
The amount drained should about equal what you need to put back in plus any that didnt make the container. Your owners maual should have a capacity for a full drain.
As far as the new mix, I use 50/50 but consult the temp chart on the back of the anti-freeze for expected coldest temps in your area.