The panels are instanced with indexing (and boolean logic). The current panels are named in a way where there's a number at the start of each name, so they appear in the correct order in the outliner (00_Bottom_Wall, 01_Bottom_Corner, 02_...) If you wish to replace any of the panels, just delete the original and rename your panel to have the same number at the start, as the original panel did - then drop your panel into the "Parts" collection. The panel needs to be exactly 0.5m away from the origin point and be oriented the same way as the rest of the panels, in order to connect to the building properly.