One benefit of the United States criminal justice system is that you have a right to appeal if you are convicted of a crime. An appeal petitions a higher court to overturn a lower court's decision. For instance, if you believe that the trial was unfair, you can petition the higher court to change the ruling and overturn the lower court's decision.
An appeal can target a portion of your sentence or the conviction. For example, if you believe you are innocent but were found guilty, you can seek an appeal from a higher court in hopes of having the lower court's decision overturned. If you believe that a sentence is too harsh, you may be able to appeal the sentence itself and seek a reduction in the penalties. Your best chance of this working is if the prison term, fines or other penalties are beyond the law's limit.