How do you change mercury outboard thermostat?

locate thrmostat housing. this is usually located at or near top of cylinder head. remove t-stat cover bolts. remove old stat, clean mating surfaces thoroughly. if yours has a gasket get a new one along with stat usually can be puchased as kit. install new stat and gasket and torque bolts according to manufacturer specs. if unsure just tighten them down in steps and evenly. be careful not to overtighten as you are dealing with an aluminum housing.. test run engine to ensure you are getting good water flow, after running awhile you should be able to touch cylinder head for several seconds. if scolding hot to touch you may have a blockage or problem with impeller. PS if you are dealing with a v4/v6 you will have a thermostat on each cylinder head. if unsure spend thirty bucks and grab a service manual. good luck and happy boating