It’s critical that the team selects the items and size of the sprint backlog. Because they are the people committing to completing the tasks, they must be the people to choose what they are committing to during the Scrum sprint.During the Scrum sprint, team members are expected to update the sprint backlog as new information is available, but minimally once per day. Many teams will do this during the daily scrum. Once each day, the estimated work remaining in the sprint is calculated and graphed by the Scrum Master, resulting in a sprint burndown chart.
from https://www.mountaingoatsoftware.com/agile/scrum/scrum-tools/sprint-backlog