Lab-One Inside : Provides links for various resolution versions.
You're looking for information on the Hk.t.rt2841p631 firmware. Here are some details: Hk.t.rt2841p631 Firmware
What of TV are you working with (e.g., Sunny SN32DAL13)? Lab-One Inside : Provides links for various resolution
The LED light should start blinking, indicating the update has started. Hk.t.rt2841p631 Firmware
You can find the "full content" (which usually refers to either a USB upgrade file or an eMMC dump) on specialized technician forums. Note that these often require a registration or a paid subscription to download:
Popular electronics repair forums and YouTube channels dedicated to TV repair are excellent resources for finding verified download links for these specific boards.