Here is some helpful information.
Addendum: An addendum is something that “adds to”. Addendum are most commonly used to describe a document that changes a request for bid or request for proposals prior to their due date. Those types of addendum do not require mutual agreement. You may also have Addenda to the agreement that describes documents that are “added to” the agreement prior to signing. In that case the agreement with the addenda requires mutual agreement.
Amendment: The word “amend” means to change, so an amendment is a document that changes an existing agreement and its signed by both parties. Most agreements require that any change to the agreement require an amendment to the agreement that must be signed in writing by both parties. There are exceptions to this where the agreement may contemplate the Buyer being able to make unilateral changes to the scope of the work.