On the back of the caliper you need to remove the two 13mm guide pin bolts - to do this you will need a 17mm open ended spanner to hold the guide pins and a 13mm spanner/socket to undo the bolts. Once they are undone you can slide the caliper outwards and use some wire to hook it to the suspension to keep it out the way. You can then remove the old pads from the caliper carrier bracket, clean it nad then fit the new pads in - put a light smear of copper grease on the backs of the pads to stop brake squeel.
You then need to refit the caliper, but you will need to push the brake piston back into the caliper - the easiest way to do this is to use a G clamp to push it back in. Keep an eye on the brake fluid level in the reservoir as this could overflow. You should then be able to slide the caliper back over the new pads. You should really use new guide pin bolts (Halfords part no. HFK810), but you can use the old ones with some thread lock if you want - but new ones are only a few quid anyway.
Once they are fitted pump the pedal a few times untill its firm, check the brake fluid level and remember to be gentle with them for the firt 100 miles to allow them to bed in.