Taking the time to learn how to change brake pads is something every dirt bike owner should do. What's also important is knowing when to change your brake pads. If they're looking thinner than Wacko Jacko after a detox... it's time to change 'em. This is generally when you're down to 30% of the pads life.
This should be a straight forward task that can be done at home swinging a couple of basic tools. I will be demonstrating using my (ex) RM 250's Nissin brake system.
Remove the brake pin plug. This is a small screw-plug that protects the brake pin from filling up with mud and gunk. Then, using a tight-fitting allen key, unscrew the brake pin and encourage the pads to drop out.
When I tried changing these disc pads, the only thing that came out was a long chain of swear-words... there was no way on earth those pads would drop out, so I had to remove the brake calliper bracket first.
While you have the brake system exposed, it pays to clean all the compacted dirt and crap out of there. Failure to do this can affect your braking performance. Once clean, apply heat-resistant lube or grease to the two brake calliper pins.
Gently push back the two pistons using a screwdriver and fit the new pads. Allow enough room for the unit to fit back over the disc.
Clean and lube the brake pin and reinsert. Screw in the brake pin plug, then reattach the entire unit to the fork.