Mass. Gen. Laws ch. 190B § 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.
Mass. Gen. Laws ch. 190B, § 7-503