Trustees can't amend a trust unless the trust says they can, which is almost never the case.
Often, the settlor/Grantor can amend the trust. Amendments must be done in the manner provided in the trust, or if no manner is stated, then follow Calif. Probate Code sections 15402, 15401, or 15404.
Note, a power of a settlor to revoke a trust in California, is also a power to amend, per case law.
I would never suggest a settlor try to amend a trust without using a lawyer. Trust amendments, particularly to beneficiaries, is a very common seed for expensive litigation. And it is very easy to mess it up; for example - persons who participate in the amendment and also benefit from the amendment are presumed disinherited in many cases; see probate code sections 21350 and 21351.
I would tell Surviving Spouse to pick a lawyer or two, and to go see the lawyers - alone - to discuss any amendment.