It is actually very easy to do with the brakes on the bike, especially if you have a lift and can get the bike up so you can work easily from the bottom.
Open the master cylinder reservoir cover and pry the pads back using a thin clean paint scraper
Remove the two pins
Slide the pads out
Slide the new outside pad in
Grease the pins (using pin grease) and put the pins in just far enough to hold the pads in place
Slide the new inside pad in
Insert the pins all the way and tighten
Bleed the brakes
You might want to check out this post...
Changing Front Brakes on Softail . It is for front brakes, but pretty much the same with the exception of sliding the pads in while on the bike.