Early termination of probation is largely up to your probation officer. It usually depends on how much time remains, how compliant you've been so far and what your plans are in the future. The first step is to talk to your PO, tell them what you'd like and why you think it's appropriate.
In Massachusetts, early termination of probation is determined by your probation officer. Determining factors in early termination include how much time has been served, and how much good behavior you have portrayed during your probation.