Timing Belt replacement

What is your question?

My wifes car has 150k miles and has not had it timing belt replaced. I know from this awesome site and all you great mechanics that it should be replaced at 90k...my question is "How hard is this to do myself(I am not a mechanic but went to school for mechanical(airplanes etc) and have worked on cars a few years) Is that the belt (outside) of the engine on the left hand side with black cover over it and if it is outside what are the difficultys to do it? help