Piece of cake unless the old sensor is seized in the hole. If it is, soak it liberally maybe several times with PBaster (Walmart etc) (not WD40) and it should come out. Much like changing a spark plug. You need the car high enouth to work from below. "Direct fit" sensors come with the origional wiring harness and "universal" sensors require you to splice four wires using a splice kit that comes with it. Universal sensors are considerably cheaper but require a little more work. You need to make sure the sensor is correct for your engine and some cars require an additional wiring harness. Check discountoxygensensors.com and volvospeed.com for a pics of the procedure.

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