By its very nature, the structure of a living trust is revocable, meaning that you can make changes to it or terminate the trust entirely during your lifetime with no consequences. It is important that you go the extra mile to include instructions in the trust document itself about how the trust can be amended. 
A formal amendment should always be prepared and signed by both the trustee and the creator of the trust, known as the creator or the trustor.
When a person passes away, however, the revocable living trust then becomes irrevocable at their death. At this point the trust cannot be updated so even a surviving spouse does not have the authority to make changes to the trust itself.
Including instructions in the trust document can decrease confusion in the future over when and how the trust can be amended. Making alterations to a trust is something that is a leading reason why many people choose a living trust document to begin with.
The circumstances you have in your life now might not apply in the future and the flexibility and control afforded by a living trust gives you the ability to evolve this document and strategy as needed.
A lawyer can help you draft or make amendments to your living trust. As part of your bigger estate plan, a living trust allows you to start making distributions and actions now rather than waiting to transfer assets after you pass away.