I'm trying to update the firmware, 384.8 to 384.8_2, but I can't load the file.
It's difficult for anyone to comment without more specific information.
When you try to update, does it give any particular error message ?
Did you unmount any USB stick or hard drive before trying to update ?
What model of router is it ?
If it is a converted TM-AC1900, it is not supported by the latest firmware builds. Asus have recently changed their code to prevent this, and this change has been carried over into the Merlin builds.
https://www.snbforums.com/threads/the-tm-ac1900-is-not-supported.48056/