Individuals under sentence (in jail, on probation or parole) can seek relief through the habeas process. Typical habeas issues include ineffective assistance of counsel, actual innocence, jail credits and any other unfairness in the criminal justice process. See my page on habeas corpus for more information. When you are no longer under sentence, it may still be possible to vacate a conviction through a writ of error coram nobis.
Sentence Modifications and Corrections
Sometimes sentences are legally incorrect or overly harsh. A sentenced person can file a motion to correct an illegal sentence which is heard by the sentencing judge. A sentence can also be modified at any point after being imposed. If the sentence is for more than three years of prison, then the State's Attorney (prosecutor) must agree for it it to go forward. There is also a sentence review process.
Probation terms and conditions can also be modified and probation can be terminated early through a motion. Parole and special parole can only be modified through a sentence modification.