To delete a single file open the action menu (the three dots at the right edge of a file entry) and select the action "Delete file". If you answer the confirmation with "Yes" the file will be deleted from the Stud.IP file area.

To delete multiple files select those files by setting the checkbox left of the file name and click the "Delete" button below the file area's file list. The selected files will be deleted after you have answered the confirmation with "Yes".