The product owner's job is to maintain the project backlog. This includes prioritising the stories based on stakeholder input and business need and breaking the more important stories into manageable chunks. They are also supposed to be available to the scrum team on a permanent basis to answer any questions the team has as they load the sprint and then work through the stories.
While the product owner sets the priority of work, they do not assign it. The scrum team is supposed to work through the work presented in priority order. The product owner can change the priority of the stories in the project backlog but not change the direction of an ongoing sprint.