I didn't change the rear brakes on time.....

Ok couple of things here - run your fingernail across the rotor how grooved is it? If there is a lot of grooves or deep grooves you should get it machined or replace it - You also need to measure the width of the rotor as there is a minimum width and you may have worn it down too far - If you don't machine or replace the new pads will not get full braking force (they may eventually bed in but not ideal) - Asking if something is easy is so subjective - I think it is easy the only trick is sometimes they can be painful to align back up to put the bolts in other times they just fall into place - You undo 2 bolts remove the caliper - push the pistons back in and fit the new pads using the spacers etc .
As you let it get so low though you need to double check the seals etc and I would clan around the pistons before pushing them back in - Also make sure you dont overflow the back reservoir as fluid may have been added if it looked low at a service - If your bike is 2 yrs old it should have the fluid replaced anyway.
I would suggest if you are not confident doing it - In this instance it may pay to go to a shop and get their opinion on replacing the rotor ad getting them to do the work.

Yeah you need to replace that like willl84 said. Also typically cross drilled rotors can't be turned anyway.

With all the engine braking on this beast I use my rear 90% of the time. I hardly touch my fronts for daily commuter riding lol