Who determines when it is appropriate to update the Sprint Backlog during a Sprint?

The Sprint Backlog is the set of Product Backlog items selected for the Sprint, plus a plan for delivering the product Increment and realizing the Sprint Goal. The Sprint Backlog is a highly visible, real-time picture of the work that the Developers plan to accomplish during the Sprint, and it belongs solely to the Developers.

The Developers are accountable for creating a “Done” Increment that meets the Definition of Done each Sprint. The Developers are responsible for planning and executing the Sprint Backlog, designing and building the product functionality, testing and improving the product quality, and delivering a potentially releasable Increment. The Developers work closely with the Product Owner to understand and clarify the Product Backlog items, provide feedback and estimates, and suggest improvements and innovations.

The Developers are also responsible for updating the Sprint Backlog throughout the Sprint. The Sprint Backlog is a plan with enough detail that changes in progress can be understood in the Daily Scrum. The Developers modify the Sprint Backlog throughout the Sprint, and the Sprint Backlog emerges during the Sprint. This emergence occurs as the Developers work through the plan and learn more about the work needed to achieve the Sprint Goal.

As new work is required, the Developers add it to the Sprint Backlog. As work is performed or completed, the estimated remaining work is updated. When elements of the plan are deemed unnecessary, they are removed. Only the Developers can change its content and order during a Sprint.

References:

Scrum Guide: https://www.scrumguides.org/scrum-guide.html

Sprint Backlog: https://www.scrum.org/resources/what-is-a-sprint-backlog

Developers: https://www.scrum.org/resources/what-is-a-developer-in-scrum