Running hot

The thermostat is in the housing right where the upper radiator hose connects to the engine. However before you replace it you might want to do a simple test to see if there might be a different issue. For this test you will need something like a balloon or rubber glove. On a cold engine you will secure the balloon over the coolant fill cap on the reservoir. Now start the engine and watch that balloon/glove. Ideally it should stay the way you left it. If however it starts to inflate rapidly you have a more serious issue, possibly a bad head gasket.

For more testing you can look at this:

https://www.2carpros.com/articles/head-gasket-blown-test

If nothing happens after about 30 seconds to a minute then you may try replacing the thermostat. To replace it you need to drain out the coolant to below the level of the upper radiator outlet hose. Then remove the radiator hose. Next you remove the thermostat housing bolts. With them out a gentle tap should free the housing. With it off you will see the thermostat inside it. To remove it you push it in and turn it clockwise to disengage the tabs. Now check the seat and housing for corrosion or dirt or damage. If it looks okay the wax pellet may be bad. A replacement part will include the thermostat, housing and seal. To install it you put the o-ring in the groove, then install it to the side of the cleaned head. If the seal falls out a small dab of silicone grease will help hold it in place. Then you put the two bolts back in and tighten to 11 ft lbs. Put the hose back on and clamp it in place.
Now refill the system with the correct coolant.
Then start the engine and see if the problem goes away. If it does then the thermostat was faulty, if it doesn't then there may be a different problem.