How do you change front brake pad on a kmx - 2003 kawasaki KX 125

Same way you do on a car. remove the wheel, take the caliper unit off and replace the pads.

1) Put the bike on a milk crate or a bike jack stand
2) Take off the front wheel, the axle comes off like a large bolt
3) Remove the caliper unit from the disc, probably allen wrenches will be needed or at least metric wrenches
4) The pads will come out with a little of force and removing the retaining springs. Take note of how the springs are attached to the pads and caliper unit. You will have to re-assemble the caliper and pads and springs
5) use a C-Clamp to push the piston back into the caliper unit. After you get the caliper unit off of the bike you'll see what I mean. Probably a 6" c-Clamp will work
6) Check the disc for wear and tear. You might have to replace the disc.
7) put new pads into the caliper and attach springs. Take your time, putting the pad springs back on may be tougher than you think.
8) reassemble the caliper and the disc.
9) Put the wheel back on and squeeze the brake. Remember you used a c-clamp to push the piston all the way back into the caliper unit. Squeezing the brake will force the piston out and push the pads against the disc.
10) Time to TEST DRIVE the bike. Go slow and check for wobble or uneven braking. If needed retighten the caliper unit, just don't over tighten the caliper and strip the threads.

Hope this helps....