Help with first time flashing firmware

Hello, I am looking for guidance on how to flash firmware on my Netgear WNDR3400v2. I downloaded the appropriate file for it (I think) from here:[]=wndr3400, but I am not sure how to flash the router. I tried 'router update' from the web interface, but that always complains that the file is incorrect or something. This will be a wired-only setup and only used to provide HDD storage to a single offline device via samba share. Thank you for your help!

EDIT: Here is the file that I downloaded and attempted to flash:

That should be the right file for the wndr3400v2 and you should be able to flash it through the web interface. I have a wndr3400v2 flashed with the exact same firmware. What is the exact error it is giving you? Is there and option to proceed anyway? Have you tried shortening the file name to something like 'openwrt-18.06.1-squashfs.chk'?