There are three revisions of your motherboards. Linked is all three CPU support lists. You will need to download and update to the latest BIOS for your revision of that motherboard. In these links you will also find links to the manuals and update files. The revision number is written on the motherboard in the front bottom corner by the PCI slot. The best supported is the AMD Athlon FX 8370E.
Rev 1.0:
http://www.gigabyte.com/Motherboard/GA-78LMT-S2-rev-10#support-cpu
Rev 1.1:
http://www.gigabyte.com/Motherboard/GA-78LMT-S2-rev-11#support-cpu
Rev 1.2:
http://www.gigabyte.com/Motherboard/GA-78LMT-S2-rev-12#support-cpu
To put it simply in order to upgrade the CPU you do the following:
- Update to the latest BIOS.
- Shutdown and unplug the computer.
- Remove the heatsink
- Clean old thermal compound from heatsink (rubbing alcohol works)
- Remove the old CPU
- Connect the new CPU
- Apply new thermal paste in a thin layer
- Install heatsink and plug the heatsink fan back in
- Plug computer back in
- Boot