In Agile we expect and accept change. As opposed to a predictive approach, where changes are constrained as much as possible, in Agile we adapt changes in real time during project run.
How does it work in practice?
Agile is recommended in projects where we expect a lot of changes due to less stable requirements. Such projects are also seen as more complex. We don’t have to know all project requirements in advance. The same is with user stories. We need to have some of them to start.
In Agile project, there is no formal process to go through to make a change. In practice, a product owner can do changes in a backlog any time it is seen as required. It means that a product owner can modify user stories, add or delete new ones, reprioritize user stories.
What is important, any change which is made, can start in a next sprint. It is not allowed to make changes in a current sprint (once a sprint is started, it is defined).