Answer:
Generally, a revocable trust is a living trust. The instrument that created the trust, sometimes called a Declaration of Trust, must set forth all the provisions of the trust including how and if it can be amended. Amendments to a trust must be attached to the original trust instrument. If the trust is recorded in the land records any amendments thereto must be recorded in the land records. Changes to a living trust are called trust amendments.
A testator can also set forth a trust in their will. That type of trust is called a testamentary trust. A testamentary trust can be amended by a codicil.