The Scrum Master is accountable for ensuring that everyone interacting with the Scrum Team, including the people on the Scrum Team, understand the way of working. In the Scrum framework, the Sprint Backlog is "by and for the Developers" - only the Developers may make changes to the Sprint Backlog. If the Product Owner discovers something new after Sprint Planning that they think is more important than other work on the Sprint Backlog, they would have to have a discussion with the Developers. The Scrum Master may facilitate this conversation.
Any such conversation should be about the Sprint Goal and how the unplanned work impacts the team's ability to meet their commitment to achieving the Sprint Goal. In the case where the team can take on the work without putting the achievement of the Sprint Goal in jeopardy, they may opt to take it on. However, if the Sprint Goal would be placed in jeopardy or if the Sprint Goal is invalidated by this unplanned work, the team may have to make a determination on what to do. The most extreme response would be cancelling the Sprint, which would be the determination of the Product Owner, but isn't possible in all cases.