There are two ways to change a stage...
1) from the submissions table (great for changing stages in bulk), and
2) from the individual application view.
From the submissions table check the box next to the applicant(s) you need to change the stage for and then click the orange "Change Stage" button that appears.
From the application view
Account Owners are able to change the stage of an application by clicking on the "Change Current Stage" button next to the stage tabs. If you hover over that button, you will see which stage is currently active.
When changing the stage, be sure to note which stages are "Application" stages, where the applicant is submitting additional information, and which are "Adjudication" stages, where you and the reviewers have access to leave ratings and comments.