This guide shows everything related to your rear LED light ( Battery powered version)
Star Union Tailight
Before mounting it, verify that your light has the 2 AAA batteries properly installed. The small tab on the rear bottom can be pushed down slightly to remove the rear cover.
Now you can close it back and proceed to remove the plastic pull tab that will allow your light to turn on. You can also remove it while inspecting the batteries.
Pressing the power button on the top will activate the light. Pressing and holding for 3 seconds will shut it down
Verify that it lights up properly before installing
To install your rear light you will need a 10mm nut driver or a 10mm wrench. There are 2 nuts on the rear of the light that will hold it in place. The screws on the LED light go through the holes on the rear rack bracket.
Whenever you need to change the battery you can use a flat screwdriver to slightly push the tab and remove it from the rack like shown here