How to rotate Ravemen CR1000 front light lens?

[Neville Bailey]

I have a Ravemen CR1000 front light for my road bike and I am impressed with it, except for the strap mount. 

I would like to mount it upside down with a GoPro adapter under my Garmin Edge mount. This will look much neater and hopefully more sturdy than the strap mount. 

Apparently it is possible to rotate the lens 180 degrees in order to keep the T-shaped light pattern correctly oriented when mounting the light upside down. I can see a little notch on the lens bezel, but I don’t see how I can use it to rotate the lens. Do I need a special tool or is there a hack out there?