Log in as Admin to the Web App -> Go To Analytics->Query Expense-> Enter the Confirmation Number (or other filter criteria available on that screen) and click Execute.
The expense report(s) will show in the area below the execute button.
Then open the expense report and change the status to Manager Approved All Levels (or any other status) and click Submit.
If you desire to reject the expense report, select update status as AP Reject from the list and click on the Submit button