Follow lower hose from radiator to the engine and where it is joined would be the thermostat housing. Behind the housing would be the thermostat.
To replace the thermostat, you would have to drain the coolant by releasing the stopper at lower end of radiator.
Disconnect the radiator hose at thermostat housing by loosening the clip/clamp and twisting the hose off.
Remove the holding bolts and the housing can come off, might require some tapping to loosen it.
The thermostat can now be removed. Note position so that the new thermostat can be installed in correct position.
Reverse procedure to install.
After installation, top up coolant and close cap. Run engine and wait for cooling fans to come on and check coolant level. Top up if necessary and take care when releasing the reservoir cap as it is under pressure when hot.