It depends on whether it is a tiled texture - ie. a 'small' one stretched to cover each of the walls of the building - or a larger texture map which includes individual images of each wall. As I am not familiar with this model, I don't know!
For a tiled texture, simply replace the original with the one of your choice making it exactly the same size and giving it exactly the same name. If there are individual walls in one big texture, then you will have to paste your own brick image over the original, avoiding windows etc. - and making sure the bricks look realistic. (This is the more difficult option, in my opinion.)
Please remember that I make these suggestions on the assumption that you intend the resulting model for your own use only and not for distribution (copyright).
Ray