You can update the stage of a participant in the Participants module by selecting a participant, clicking its Actions menu (the three dots), and selecting the "Update stage"; or by navigating into the participant details, and then to the stage tab.
A pop-up window will then appear, allowing you to change the stage of a participant.
Make sure to save your changes by clicking Apply. You will then see the changes appear in the appropriate column in the participant list view.
In order to view or update the stage of a participant from within participant details, click on the participant you'd like to manage and navigate to the Stage tab. Once there, click Edit Stage or on stage name, and select the new stage from the dropdown menu. Your change will be saved once you click Apply.