Hi,

Not sure why a garage would not do the job but I changed brake fluid by extracting the old brown fluid from the reservoir with a syringe.  I replaced it with Dot 4 brake fluid and repeated the process at a later date to remove more of the old fluid. This will not get all of the old fluid out but should leave you with a reasonably clear golden liquid. Just make sure you do not get any fluid on the bodywork.

Alan