Mitsubishi Pajero Heater Matrix Problems

changed therostate and ive still got no heating,blower works fine.blowing cold air all the time,even when dial is on hot.sombody said give it a flush !! how do i do this ? any suggestions please ?

Hello. the heater blows cool air inside. what can i do. the temperature knob is at maximum hot. the fan works well but i receive cool air inside what can i do to repair it by myself? thank you for your help