Once you have downloaded the file (often in a .rar or .zip format), extract it to get the .bin file. You can then update using these methods:

Use the Dahua ConfigTool on a PC to find the device on your network and push the update remotely.

Check the sticker on the bottom of the device or the System Info > Version menu in your DVR settings.

A "16MB" firmware file typically refers to the size of the flash memory on the DVR's motherboard, a common spec for many 8-channel units using HiSilicon SoCs. How to Find Your Firmware

If your device is connected to the internet, you may be able to enable Cloud Update via the mobile app iDMSS/gDMSS or the DVR menu to skip manual file handling.