If you wish to reset the LG G3 D855 then you will need to follow the following instructions which are standard on this range of LG mobile phones. Note that there are two options when doing a hard reset on the LG G3 D855. There is the manual way of doing this and also the software process to do a factory reset. You do this at your own risk as it deletes all your settings.
Before you start the reset process you would be better off making a backup of your mobile phone first. Once the data has been deleted you cannot recover these settings again. Always ensure that you have either backed up the data or copied the settings down with pen and paper. This includes your APN as well as internet settings as these will also be removed during the process. The factory process deletes all your add on programs as well which did not come with the phone.
Hard Reset will erase all of your data
All described operations you are doing at your own risk.
Factory reset via Menu
Factory reset via Buttons