How does a person have a felony reduced to a misdemeanor?

My sister was arrested for something minor when she had just turned 18. She had never been in trouble...not even as a juvenile or traffic ticket. She was so young and naive at the time. I remember her being so terrified when this happened. She was incarcerated for 4 days and was sentenced to 3 years probation, restitution and was slapped with a federal felony. It has now been 8 years since the incident and now believes she was dealt with too harshly. She hasn't even been in trouble since then! She was even removed from probation 1 1/2 years early for good behavior and has really advanced in life! Can someone help and give me advice? Can she have the charge reduced to a misdemeanor? If so, how do you go about doing it? Thanks!