Using the pet **** in bottom tank of radiator, drain out a couple gallons coolant, enough that level drops below the thermostat location.
Remove upper radiator hose where it attaches to the engine cylinder head.
Place thermostat onto the cylinder head, sensor part of stat down into the head.
Replace radiator hose and tighten hose clamp. Get a second hose clamp, place it above the thermostat and snug-up slightly to pinch the hose. (this will prevent stat from being pushed up the hose. Note if your hose is enlarged where it fits on the head with reduced diameter above the stat (as original), you can skip the second clamp)
Repeat for the other cylinder head.
Refill the radiator with coolant just up to where radiator core is covered. (leave room for expansion into the expansion tank above the core)
Run the engine and check for leaks. After engine has run at temperature, recheck coolant level in the radiator.
BTW: 160 degree stats were original. With todays coolants that don't evaporate, 180 degree is better for burning off contaminants.