Most engines have the thermo housing on the other end of a radiator hose.
Its the housing with three 10mm spanner size bolts...they're 6mm bolts and the nominal torque is about 11-15 Nm.
Only do this when the engine is cold and remove the radiator cap.
Drain the radiator and drain the block.
Remove the above 3 bolts and replace thermo and Oring.
Fill with water and start engine.
Do dynamic flush of cooling system.
Be Very carefull of airlocks as temp maybe stable until you drive car and overheating will occour.
So it is better to manually bleed cooling sys while engine is at operating temp and danger of burns has increased.
If not completely confident in above procedures ...better to let the shop do it.