Users typically seek an update file (UPD) for the following reasons:

If the USB port is dead or the bootloader is corrupted, technicians use a UART-to-USB tool (TTL converter) to communicate with the Mstar chip directly via command line (using tools like MstarTool or SecureCRT). This requires soldering and technical knowledge.

Contrary to intuition, you do not copy the UPD file to a USB flash drive. You copy it to the root directory of your Windows hard drive (e.g., C:\msd338_firmware.upd).

Then, you use the manufacturer's flashing tool (usually ISP_Tool_V4.5.exe or CVT_Flash.exe). If no tool is provided, the MSD338 has an "FAT12 emulation" mode.