How do I underclock/undervolt an fx6300 on a Gigabyte GA-LMT78-S2?

The FX-6300 doesn't have great performance to begin with, so underclocking it is just going to drive performance even further down.

Are you sure you're having overheating issues? What temps do you have at idle/under use, & what app are you using to measure it? If you haven't already, have you downloaded & used AMD Overdrive to double-check the thermal margin at idle/under use? It works in reverse to normal temp measurement, in that instead of measuring the temp it measures how much room you have before you hit your max temp; with the margin, higher = better.

If there is some overheating occurring, undervolting/underclocking the CPU is a very drastic step. It would be better to check a few things:
-- check your case documentation to see if you can install any more fans in your case, & to make sure that your case fans are installed in the optimum configuration.
-- make sure the airflow to & from the case isn't restricted. For example, if you have a computer desk with a compartment for the case, make sure that a) the front is either open or has the door open when operating it, & b) make sure the rear is 100% open, no backing or anything solid behind the case whatsoever. If possible, consider moving the case so that it's sitting out in the open (either on top of the desk or on the floor, just make sure that the intake & exhaust fans have unrestricted access). Also consider where you're using the PC; using it in a room that's consistently hotter than the rest of your place will result in higher operating temps because the ambient temp starts off higher. By the same token, putting your PC on or near a vent may work in the summer when AC is blowing out, but is a very bad idea in the winter when the warm air comes out.
-- If it's been a while since you last did it, consider reapplying the thermal paste. Old paste doesn't work as well as newer, & too much/too little paste is also bad for the CPU. Also take the time to clean any dust out of the CPU heatsink & fan.
-- If all else fails, consider replacing the stock cooler with something better. Doesn't have to be expensive, you can get decent ones for $20-50USD.