This is an unofficial version of a Massachusetts General Law.
Section 7-503
A trustee may resign by an instrument in writing delivered to the transferor or to the transferor's guardian or conservator, if any. A trustee may be removed by the transferor by an instrument in writing delivered to such trustee. If there is more than one person serving as trustee, a vacancy need not be filled, and until a successor is appointed the remaining trustee or trustees may act alone. In the event of a vacancy a successor may be appointed by the transferor, if legally competent, or as the transferor shall have provided by a written instrument, and otherwise by the transferor's guardian or conservator, if any, and if none, by the transferor's heirs presumptive, and such appointment shall become effective upon acceptance.