How difficult is it to replace a thermostat on a 1988 cadillac seville? Where is it located?

They're not too difficult.
On the engine end of the upper radiator hose (2) it the housing (7) that contains the thermostat (6) and gasket.
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Start by obtaining the correct rated thermostat for your car. Have both a new t-stat and gasket/seal plus a 50/50 mix of fresh ethylene glycol coolant. You shouldn't need much if just replacing the t-stat.
With the engine overnight cold, remove the two bolts that secure the t-stat housing to the intake. Move the housing with hose attached aside. Remove the old t-stat and clean the seat to ensure a proper seal. Reinstall the housing and tighten the bolts. Refill the system and check for leaks.
It may be necessary to drive the car to purge all the air from the cooling system so keep an eye on the coolant reservoir bottle for the next couple days.
Good luck!