thats proberly one of the easiest job's on a disco mate..
take your wheel off(start at driver's) and get it up on good axel stand's
open your fluid resivour(for when you press the pistons in the breaks back)
take out the split pins on the driver's side then using either a break compression tool or a strong flat bladed screwdriver force the pad's backwards towards the caliper(try not putting excess pressure on the disc its self thou as you may score it)..
once you have pushed the cylinders back in you can lift the old pads out.
give the disc a wipe to remove any accessive grease or gunk then place the new pads in and replace the spring retainer and split pin's...
pump the break to put it back in to place check and top up your fluid if required and replace the wheel..
repeate this on the other three in order of passenger front driver's rear passenger rear..althou any order will still work the same..
once your new pads are in take it steady and use the breaks gentally to bed them in..
two bolts to remove the caliper..i would also remove the line retaining bracket(another two bolts) save's snaping or bending the fixed lines(use a zip tie to hold it up on the spring but don't force the pipe's)...
you should see the hub and disc they need splitting.. you have to remove the hub assembely..
undo the retaing bolts for the disc and useing a soft mallet tap it off the wheel hub.
refitting is the reverse just make sure it's lined up correctlly with the bolt hole's and that it is sat flush with the seat on the wheel hub..
you then need to check it's run off with a dial gauge..
this isent a simple job like the pad's and if you are new to home mechanics i wouldent advise you try this without someone with experience.