you might want to get in contact with nissan, i'm not to familiar with there warranty, but with toyota warranty, you can get the discs replaced free for just having rust on the bits that dont even touch the pads just cos it doesnt look good, and if the discs scored then they might get away with a faulty pads claim, its worth a try mate, most people dont realise who much warranty work they can have done on their cars,
you should have 2 bolts on the caliper (probably 12mm 13mm or 14mm) take these out and then you can remove the caliper, take the pads out and check them against the new one, check that the if the wear indicator has to be used of the old ones or is already on the new ones, also check the pad material cos some pads have a chamfer at one end that needs to be at the top, then you will find 2 17mm bolts on the caliper cradle, these will be tight so you might need some leverage, take the cradle of and then unless there are any bolts holding the disc on you should be able to remove the disc with a little persuasion from a hammer (pref rubber but if your changing the discs it dont really matter) put the new discs on (and i usually put on one of the wheel nuts to hold them flat) then put the crable back on (i think the torque is 108nm/80ftlb but nissan should tell you) then put some copper grease on the ends of the pads and the contact areas of the back faces (making sure not to get any on the friction material) and put them in the cradle, then squeaze the caliper piston back in and fit this, put a little bit of grease on the 2 caliper bolts and tighten these, and then your done