My 2002 Prius maintenance manual states that the coolant in the inverter should be replaced at 30,000 miles (along with engine coolant). The manual says the inverter coolant is an ethylene-glycol type. It does not indicate if it should be a 50/50 mixture as is the typical mix for engine coolant.
The Prius repair manual volume 2 in section HT-6 describes the drain and fill procedure. It involves removing a drain plug on the side of the transaxle to drain the reservoir, inverter, and transaxle. Refilling requires the attachment of two small diameter hoses to bleeder plugs located below the front cover of the inverter and placing the open ends of the hoses in the inverter reservoir. Coolant is added to the reservoir and coolant is siphoned thru the hoses. A procedure must be followed to run the electric inverter water pump to bleed air from the system. It does not appear to be a difficult procedure, but as with changing the engine coolant, you must follow a step-by-step procedure to bleed air from the system.