There is no way to entirely eliminate all risk when upgrading the firmware on any device. However, you can minimize the chance that something may go wrong on this router by factory defaulting your router first, upgrade the firmware, and factory default your router one last time. After that is complete manually reconfigure your router with its previous settings. Be sure that during the upgrade process when you have a progress bar going across the GUI of the router that you do not hit anything on the browser. Just let the browser sit while it completes the process and then reboots.

I would not worry too much, I have upgraded the firmware on this router many times and I have yet to have one turn into a brick.