There are 3 ways how to update firmware.
This is the only way how to update both bootloader (lk.rom) and ROM (tcc8900_mtd.img). Bootloader is device specific and contains basic program, configuration (mtd, wlan, etc), and splash screen (on HSG devices butterfly image from devianart). ROM contains disc images for all parts (kernel, system, revovery, etc.).
To enter FWDN mode it is needed to enter FWDN mode, this is device specific (check product page for your device for details). Mainly it is combination of keys + POWER or holding touchscreen + POWER. Connected charger is needed to enter FWDN mode. Some devices (for example HSG X5A, not X5A-G), has FWDN switch on board. Other devices are likely to have some way to enter FWDN mode too by shorting specific pins on PCB.
Android Debug Bridge allows to flash or erase each mtd block.
adb shell flash_image boot nand/boot.img
To use this functionality, it is needed to have custom ClockworkMod installed. Factory ROMs doesn't have this fuctionality. On devices which doesn't have any key combination to enter recovery mode, it is needed to force it by
adb shell reboot recovery
Notice: Do not use FwTool, this tool is included in TCC8803 ROMs and using it causes deleting EEPROM of touchscreen controller.