Hello, and welcome here..
Assuming it is built in the traditional way, a chisel is the first thing that comes to mind. Block by block. Don't forget safety glasses!
At the same time, it can be both difficult and time-consuming to get the existing blocks in a condition that makes them reusable, and to get them to sit straight and attach properly..
Simple standard-sized glass blocks are now sold for under 20 SEK each.