My understanding is that there are two coolant loops: engine and inverter. To service the engine coolant, drain the radiator and overflow reservoir and refill. To service the inverter coolant, find the drain plug below engine and drain and refill. No need to post about air pockets, etc. Think I got that figured out: Is this the correct procedure for both loops?
Regarding the engine, you might also open the drain cock in the engine block to see how much fluid comes out, after you have drained the radiator first. Regarding the inverter/transaxle, the coolant drain plug is located on the transaxle housing. Do not confuse this with the drain plug for the transaxle ATF, or you will get to replace the ATF as well. I believe that other posters have posted nice photos showing the locations of those drain plugs. Or you can obtain factory repair manual info at techinfo.toyota.com