Retain a criminal defense attorney who can file a motion in court requesting that the Court order the release of the same for retesting. If you cannot afford a private attorney then apply for a public defender. Good luck.
You need to immediately speak to an experienced criminal defense attorney who is familiar with the institution where the drug test occurred. You didn't mention in what context the urine test was conducted, what sanction was being imposed or whether there is an administrative procedure within the prison to appeal from the dirty urine. However, an attorney familiar with the institution should be able to tell you what course of action is available-whether it is administrative or directly with the court.