Updated: Mar 2
Although the team has the final authority regarding which stories to include in a sprint, the PO can still influence which stories are added to the sprint.
Re-prioritization of the stories
The PO can use his power to re-prioritize the backlog and change the sequence of stories. When he does, the team must work on the highest priority stories first. One thing to remember here is that although the team is obligated to the priorities set by the PO, they are still limited by their velocity and capacity. Therefore, the PO can only prioritize the items in the product backlog and hope the team estimations will allow them to commit to as many stories as possible.
Story breakdown that changes prioritization
Story break-down is another way the PO can influence the team to change its decision. Let us think about a story that the team estimated at five-story points. The PO might decide that some aspects are not necessary. Therefore, he splits this story into two smaller stories (P1 and P2) with a different priority levels. This allows the team to commit to P1 and take another story from the product backlog that does not exceed their maximum velocity.
Changing the scope of a specific story
The PO can choose certain stories and reduce their scope (by removing specific items related to their acceptance criteria), requiring less coding, testing, and research activities. This is usually done for stories estimated at five-story points (or more) and allows the team to commit to more stories than they had initially planned.