The Backlog items on the top of the Product Backlog are the Backlog items who are candidate for the next sprint (iteration).
As such PTLs need to provide clear priority (to your Team) and always move on the top of the Product Backlog the items for next sprint.
To prioritize your Product Backlog, simply select and hold a backlog item, and move it up or down (watch the 4 seconds video).
It is the responsibility of the PTLs, to always "GROOM" the Product Backlog. Meaning, have the top Product Backlog always prioritized and properly defined so the team can proceed with effective sprint planning.