Citylite is a nice and effective danish bicycle light.
It has the good property to keep the light intensity at max until the bitter end.

But if you want to change the battery, you might need this instruction:

The original idea is to push both sides of the glass, and then pull it out,
but that can easily break the thing, especially in cold weather.

So, use a knife as a tool to bend the white plastic outwards.
The next picture shows he locking mechanism.

NB Be careful not to lose the black rubber button!.

When taking out the batteries, first push out the upper battery,
and be careful not to hit the components on the board when pushing out the lower battery.

The batteries are 2 pcs CR2032, mounted in an unusual way (in parallel).

...Happy cycling!