Changing water pump - do I need to drain all the coolant?

18 at Walmart Asian red works perfectly labeled as ok for Prius. Coolant wears out ya know!

 

Use a clean container and drain the fluid from the radiator and refill with it.
You'll have to add some new coolant probably.
You should only use the Toyota coolant. Even if any other says it okay for Toyota,
it doesn't mean it is. It may turn the coolant into a jell.
And the cost difference in extremely minor.
Pour slowly, very slowly, a little at a time. You can hear it bubbling in the engine.
And fill to the "B" line. Start normally, and wait until the engine shuts off, then check the coolant
level. It's probably at the full line, or close. Add some if you need to. Then drive the car close
to home for a few miles to get it up to temperature. Then park on level ground and clean up.
Check the level in the tank, it should be at the full mark, or just above.

 

These steps are a little different in the repair manual: after filling to the B line, they have you start in maintenance mode, so the engine doesn't shut off early. You want to let it keep running until it is fully warmed to thermostat-opening temperature and coolant is flowing everywhere (the bottom radiator hose becomes hot, or you see the cooling fans come on, or you read at least 85–95℃ (185–205℉) on an OBD reader). Then let it run that way several minutes more.

Then turn it off and let it all cool down. The level should end up at the full line (or very close) when it is all back to room temperature.

Without using maintenance mode, the engine is likely to start, run just up to a minimal operating temperature, and say "hey, I'm a Prius" and turn off. That can be too early to get all the air out.

 

Info regarding coolant change and maintenance mode:

 

Which offers the comically misplaced "HINT" that the block drain "plug is on the back of the generator on the exhaust manifold side."
What generator? In a Prius?

 

That reminds me: I would not bother with that block drain; it’s a pain to get to, and yields literally a few tablespoons.