2006 Alfa Romeo 147 Problem with the engine temprature

Hi, my name is Haik, I am 32 years old and I am from Greece. My car is Alfa Romeo 1.6L twin spark.
The problem is that when I turn on the heating system the engine temperature begins gradually to
fall down the 90 degrees (celcius I assume) until 85-87 degrees. I assume that this happens because
the thermostat has possibly stuck open or closed I do not know. The question is first of all if what I described is the cause of the problem and how I can fix it by myself. The sole reason is because I like to learn and fix my car alone. Thank you for all and I am looking forward to your replay asap.