If you have already installed OpenWrt and like to reflash for e.g. upgrading to a new OpenWrt version you can upgrade using the mtd command line tool. It is important that you put the firmware image into the ramdisk (/tmp) before you start flashing.
Terminal Upgrade Process
Login as root via SSH on 192.168.1.1 (or whatever you configured it to).
Use the following commands to upgrade:
Numbers 0-3 are Ports 1-4 as labeled on the unit, number 4 is the Internet (WAN) on the unit, 5 is the internal connection to the router itself. Don't be fooled: Port 1 on the unit is number 3 when configuring VLANs. vlan0 = eth0.0, vlan1 = eth0.1 and so on.
Each blue cover is held in place by 2 bracket-like clips in the front (LEDs side) and 2 round brackets in the rear (network ports side). To remove the top cover (the one with the button going through it), pull the back of the blue cover away from the router and towards the front (in such a manner that the front clips act as pivot). The round clips should release relatively easily. Then, gently shake the cover left and right until the angle bracket release from their sockets. To remove the bottom blue cover, you first need to pull the rubber feet towards you (there is a spring mechanism that will allow them to be pulled) enough so that you can rotate them 180 degrees. Once the feets are out of the way, proceed in the same manner as for the top cover.
Once the blue covers are out, you will need to unscrew 4 T10 secure Torx screws located in the corners under the bottom blue cover.
After that, you will need to remove the black front part. In order to do so, you will need to squeeze all 4 pairs of arrow-like clips attaching it to the main (grey) cover. Using longnose pliers, gently squeeze one pair together and tilt the black part slightly such that the clips won't clip back again. Repeat for all 4 pairs of clips.