Hi Ahmed.
If you need to get the hearter matrix out (which I presume you do), you will need to remove the fasteners indicated by your Red arrows. If they are turning can you get to the underside of the heater unit to hold the nut? The plastic rivet type fasteners are re-usable, but if you happen to damage them, I have some that I can send you. Plenty in the archives about where to get a replacement matrix from.
Give our regards to Margaret.
Stuart