Update the firmware of a TP-Link TL-WR842ND the easy way

First connect your computer to the LAN 1 port of the router and set your ip to a static 192.168.0.66. Then start a tftp server, on Mac it is easy since there is a built-in one available.

sudo launchctl load -F /System/Library/LaunchDaemons/tftp.plist
sudo launchctl start com.apple.tftpd

Move the bin you want to flash to /private/tftpboot/wr842nv2_tp_recovery.bin and fix permissions. If you are lazy just do 777. Plug the router into the power source while holding the reset button pressed for 5-10 seconds. Switch your network settings back to DHCP and wait to get an IP assigned.