There is no single "official" download link for CVD1810 firmware, as this identifier typically refers to the MCU (Microcontroller Unit) version of various generic Chinese Android car head units. These devices are manufactured by different companies (like XY Auto, ZXDZ, or Joying) and use different processors (such as AC8227L or MT8163), making firmware highly specific to your exact hardware.
ESC or Ctrl+C during serial boot to interrupt U-Boot, then flash via tftpboot.Transfer the File: Copy the firmware file directly to the root directory of the drive (do not put it in a folder). cvd1810 firmware link