How diffulcult to replace spkplugs. What is s.o.p.

At that mileage these engines usually develop some leaks at the cylinder heads and valve cover gaskets. Also if not done already, the timing belt and all related componets as well as oil seals need to be replaced. As for changing the plugs that also should be done by a mechanic with the proper tools. Problems with cross threaded spark plugs if not done correctly!