Download the Stock Firmware for Your Specific LG G3 Model.
Copy and Paste the MegaLock.dll File to C:\LG\LGFlashTool (assuming that is where you installed it).
Navigate to the Extracted LG FlashTool Folder.
Do Not Run LG FlashTool When it Finishes Installing.
Double Click LGE Mobile USB Serial Port.
If You Can’t, Download the LG USB Drivers and Install Them(and then restart your PC).
Make Sure You See LGE Mobile USB Serial Port.
Locate the COM & LPT Ports Option and Expand it.
Wait Until You See a Firmware Update Screen.
Plug in a Micro USB Cable While Still Holding the Volume Up Button.
Press and Hold the Volume Up Button on the LG G3.
Plug in a Micro USB Cable into Your Computer.
You will lose all of your data and it will be like you had just pulled the phone out of the box brand new. Downgrading your version of Android is essentially like a factory reset.