If you manage to locate a specific firmware file (typically an .epk file) from a regional LG support site or a trusted archive, the manual update process is as follows:
: The device was originally designed to receive firmware updates automatically via its built-in Wi-Fi or LAN connection. If the device is still functional, you can check for updates in Settings > Support > Software Update . Manual Update Procedure (USB)
How to Update LG TV firmware with USB drive to Fix software errors