It sounds as if you are mustering your resources on this matter, bravely.
Be cautioned: a TV may have dangerous voltages inside it. With modern (i.e. flat-screen) types of TV, as opposed to the older 'tube' or 'CRT tube' -type TV, the hazard is somewhat less. But, to prepare a TV for being opened and worked on, you should
A. unplug it from the power / mains / outlet.
B. turn it On with the power switch (even though it is unplugged - this will drain voltages to some extent, that may persist inside it.)
C. leave it that way, unplugged with no power source, but turned On, for at least 2 hours before opening it.
D. Yours is an LED TV. That should be enough - there is no High Voltage section, no 25,000-Volt tube cathode to worry about... just don't use your bare fingers on any capacitor leads! and you should be safe enough after the steps A through C above.
If the backlight is partly On and partly Off, unfortunately (since this is an LED TV) you might have an expensive repair case, one where replacing the TV would be both easier *and* less expensive than repairing it.
The backlight being partly On (and only on part of the screen?? that doesn't sound like a backlight problem. It sounds like a circuitry problem or power transmission problem. Or at worst, the whole display panel is defective, in which case replacing the whole TV would be the better solution...) is not a good sign, but might be owing to defective power supply to it.
(Which might be owing to dirty or corroded connectors...)
So I have to ask, before advising you further:
- How old is the TV, measured from date-of-unboxing ?
- How damp or humid an environment has it been in since it was unboxed and you started using it? damp/humid conditions encourage corrosion.
- As for how dirty or dusty it has gotten... I can't ask meaningfully. When you open it, you'll be able to tell for sure.