Thanks Florin!i will change thermostat only but i am not sure about steps.plz correct me if i am wrong:
1.draining of coolant
2.removal of return hose pipe from radiator and engine
3.Open supporting clip for bottom coolant hose and take off top part.
4.Unscrew hose at thermostat housing.
5.Removal of thermostat housing by removing two bolts.
6.replacing thermostat and re installation in reverse order.

here i have two questions:
1.can i use AcDelco coolant rather than G11?
2.what is the type of bolts at thermostat housing, will i need any special tool to remove these?