Once a sentence has been given, there are still ways to have your case heard for a second time in an effort to have your sentence reduced or sometimes even dismissed entirely. To do this, you must prepare for an appeal with the lawyer that you had before or a completely new one. They can point out parts of the case that could be examined more closely in an appellate court.
The first part of the appeal process is submitting a written opening brief to an appellate court in order to apply for an appeal. In the United States, everyone gets to have a chance for appeal at least once, but sometimes more times if a case is big enough to go to the Supreme Court. The brief is the document explains why an appeal is necessary for this particular case.