How do you change thermostat on Chevy optra?

its simple

If you don't know where is it, you can follow the upper pipe out from Radiator to engine. so its that part connecting to water neck.

Remove water neck the screws.

you will find the thermostat embedded in the neck. just replace it with new one..

Make sure to Clean the housing, the cylinder head mating surface and the screws with a small brush. then put thread sealer on the edges.