You can replace the failed LED if you have very good soldering skills, but most people will replace all the strips (these two strips cost £16 on ebay). This ensures a longer life because if one LED has failed, the next failure won't be far behind.

Be careful removing the old strips because they are normally well glued on. I wear gloves because it's easy to snag a finger on a sharp edge. Start at one end with a blunt knife or similar to cut the glue then twist or lever the knife a little to ease the strip away from the metal chassis. They should come off in one piece.

Fix back the connector board with the three screws. Partially peel back the adhesive covering from the new strip at the connector end and connect it up to the board. Keeping the strip between the guide pips, gradually pull off the adhesive cover so the strip sticks in place. If happy with positioning, you can press it more firmly in place.

Reassemble the rest of the TV, and it should all be working OK as in image.