Do I need to drain coolant before I replace?

Learning as I go.

Thanks

 

I wouldn't drain coolant unless it's due for scheduled replacement.

Let car sit still a few hours so it is nice and cool.
Relieve pressure on the system-- take off radiator cap then reinstall cap.
Unscrew sensor. Quickly install new sensor, minimize loss of liquid.
If it's the one on the back of the head, it's tricky.

If it's the one in the bottom of the radiator, you might be better off draining out the antifreeze first. I don't, I just make sure I don't get drenched.

Top off cooling system and get trapped air bubbles burped out.