Use a utility like WinRAR or 7-Zip to extract part1.rar . This will automatically combine all parts into a single folder containing a .bin or .img file.
This model is a high-performance 55-inch television featuring: 4K Ultra HD ( Operating System: Android-based with built-in Wi-Fi. Download 35018484 3840x2160 Nikai UHD55SLED3D part1 rar
Firmware files are specific to the mainboard number and panel number . Using the wrong software can permanently "brick" your device. If you are unsure, consult the official Nikai Support Page or a professional technician. Use a utility like WinRAR or 7-Zip to extract part1
Move the extracted firmware file to the root directory of the USB drive (do not put it in a folder). Installation: Turn off the TV and unplug it from the power source. Insert the USB drive into the TV's USB port. Firmware files are specific to the mainboard number
Press and hold the on the TV (not the remote) and plug the TV back in.