Armbian is the most reliable source for Linux firmware on H6. Go to armbian.com/download and search for your device (e.g., "Orange Pi 3 LTS"). Armbian images are shipped as .xz compressed .img files. These are bootable from SD card or eMMC.
Use a toothpick or needle to press and hold the (usually hidden inside the AV port ). allwinner h6 firmware download
Once you've located the correct firmware for your device: Armbian is the most reliable source for Linux firmware on H6