Is it possible to change the battery in a 54604 light brick?
There's a YouTube video which explains how you can replace the battery in that light brick.

You can also find the owner's explanation on the Eurobricks forums.

Quoting from that site:


  What you'll need:
  
  
  ExoForce Light brick (obviously)
  A small screwdriver (or other item with a long, thin shaft) (my s'driver was 2mm (0.07 inches) thick)
  A small 3V battery.
  
  
  Put the point of the screwdriver into the hole in the brick (where you'd normally put the fiber optics cord).
  Using quite some force, lever the screwdriver until the 'lid' pops up at the front.
  Insert the screwdriver into one of the small gaps at the side where the 'lid' pops up, and lever the lid up again.
  the lid should come right off. From here, you can shake the brick and the battery should fall out. Simply slide the new battery in and
  click the lid back on.