The firmware is typically distributed as a compressed archive containing an image file (often named aml_s905_q201_7.1.img or similar) and installation scripts.
If your device is still functional, you can try an over-the-air update: MXQ Pro 4K [Android] - 4PDA i--- Mxq Pro 4k Firmware 7.1 2 Download Sd Card
Here's a step-by-step guide to updating your MXQ Pro 4K firmware using an SD card: The firmware is typically distributed as a compressed