how difficult is spark plugs replacement?

It really depends on how much work you have done on cars, if this is your first time, well that is a VERY EXPENSIVE engine to learn on, so if you cross thread a plug, are you willing to pay about $3000 to fix the cylinder head? If so, then go ahead, if you feel uneasy, pay the 1.5 hours + parts to a Technician to do the job and let him/her take on the responsibility. Hope that this makes it clear.