While many people associate this with the U.S. Constitution, a Constitutional amendment is only one form of this action - which essentially changes, alters, modifies, or clarifies a court order, a complaint, a law, or any other type of legal document.
To cite a an example with which you should be familiar, the Bill of Rights consists of the first ten amendments of the U.S. Constitution, which is supposed to be the ultimate written authority on legal matters in the United States of America. The Framers and Founders felt these basic rights were so important, that even though these rights had not been specifically denied by the Articles of the Constitution, they should be legally defined in writing.
Amendments may also be added to legislation in Congress as well as state legislatures. While the normal purpose here is to clarify and specify a point of proposed legislation, members of Congress have abused the amendment process over the course of history by attaching "riders" or extra language to a bill that has nothing to do with that bill's primary purpose.
Amendments may be made to employment agreements and or contracts as well. For example, if a company has been producing a product using a substance that is later shown to be harmful, an amendment may be added to the employment agreement requiring the company to provide protective equipment at no cost.
Amendments may also be repealed or rescinded - in other words, cancelled. Rules for this vary from one state to another and usually depend on the nature of the document.