Its very simple really. I don't know what kind of case you have, so how ever you'd get to the LEDs for power and HD, get there, ensure the power is shut off, cut the wires keeping in mind you will need to know which ones are power or HD and with one is positive and negative, strip the wires you just cut, solder the new LEDs to the wires and wrap with electrical tape. That's it.
Well that's it for how to do it, but do make sure that the LEDs are rated for around 3.3V being that's the amount of power running from the motherboard.