How do I upgrade my LG LH 20 lcd tv firmware

As far as I can see on the support page, here: Product Support
there is no firmware upgrade. An upgrade is only made, when several people have a issue with there device, and it can be solved by upgrading the firmware. Ir is NO SOLUTION for real defects.
You even can't upgrade if the device is not working correct.
It needs to run a short program, put is a small part of the RAM, and during the upgrade it can't lose power, because the little program has to flash (put new software in) the electrical erasable memory.
When something goes wrong during the update, nothing works and you need to visit a service centre to put things right.
As long as your TV works fine, don't update. When LG really does have an update, they will email all registered owners and carefully explain how to do the upgrade.
To be sure about any updates, register your TV on the support site.
When you have a problem with the TV contact LG or a certified repair centre close to you.
It would not be wise to try to change something on your TV, as long as you are not 100% sure it would improve the TV.