Flash DIR-835-A1 “factory” OpenWrt image file (e.g. openwrt-ar71xx-generic-dir-835-a1-squashfs-factory.bin) via D-Link firmware upgrade in web interface. *NOTE* the process completion is not obvious so give plenty of time and do not be confused as the LED power light will end up being AMBER instead of GREEN as on stock firmware; LED functionality can be adjusted through 'LUCI > System > Led Configuration'.
Note that default D-Link IP address is 192.168.0.1 and OpenWrt will default to 192.168.1.1. Make sure your PC obtains a new IP address from DHCP (or use static) after installation. Initial connection must be over ethernet (you should flash over ethernet anyway).
Telnet to 192.168.1.1 and set a root password, or browse to http://192.168.1.1 if LuCI is installed. The file linked above (14.07) includes the Luci web interface, but note that trunk builds will require you to manually install it (see luci.essentials-tomerge). WiFi is disabled by default as well, as it is with most other OpenWrt builds.
Remove the two (2) back rubber feet. (closest to the ethernet ports and antennas)
Unscrew the two (2) screws underneath the feet.
Pry case halves apart from back side to front side, carefully.
To remove the board from the casing:
Unscrew the two (2) front (inner) screws and remove the LED shield.
Lift board from front and slide out in a forward motion.