The instruction manual offers no direct method of changing the battery despite being in several languages.. There is the part that implicates am involved method of getting to the inside:
For servicing and repairs, the removal of the access screw on the back of the watch gives access to the stem release-catch; after removal of the crown and stem, pressured air is inserted to blow-out the crystal. Thereafter, the entire movement, together with the dial and hands are easily removed from the top. Experienced watchmakers with the necessary equipment should only perform this operation and the removal of the access-screw by any other persons without the required expertise is highly discouraged.
Unfortunately, this is the way to change the battery. If you look up how to un-case an Omega Seamaster Cosmic 2000, you may find some people mention using pressurized air to pop the crystal out and remove the movement to replace the crystal. I must add that I do not have experience with this particular Braun watch, but if its a ronda chrono quartz, it must have a battery; and changing that battery is of course part of the servicing mentioned in the instruction manual. Its gonna be tricky, but hopefully you'll hear from some others on this site---by the way WELCOME!!
J