You can change the status of files by clicking the Set Status button in the top right corner of the file viewer. Most files have three possible statuses; In Progress, Changes Requested, and Approved. Changing the status of a file will also change the status on the thumbnail view of the file.
Pro Tip: Once all files within an Activity folder have been marked as Approved, the folder will receive a checkmark to indicate that the entire folder has been approved. This helps you quickly understand what parts of the project have been completed.