The leaving water temperature is set in the plant using a SetpointManager:* object. Place a set point manager at the outlet node of the plant. Typically the water temperature is held fairly constant so the SetpointManager:Scheduled object is commonly used. Other managers can be used, for example SetpointManager:OutdoorAirReset. The entering water temperature is the temperature of the water entering the plant (plus possible pump heat) and is a result of the components used on the demand side of the water loop and how those components operate. Control of the return water temperature is not typically used.
Could you please retag your question using existing tags and the software you're using? Also a little bit more of context, which object exactly you're using, etc, would help.