The core firmware, often named something like SKR8503_100.bin or T.RD8503.bin .
The "1366x768" in your query refers to the specific display resolution of the panel the firmware is intended for. Download RD8503 Sony 1366x768 rar
Power on the board. A light on the keypad or board will typically begin flashing (often red and green or rapidly blinking red). The core firmware, often named something like SKR8503_100