Using the Recent Files list you can quickly switch between different files - even when they are located in different folders. This popover also has a button to close the currently open file.
(1) You can show a list of Recent Files by tapping on the "recent files" button or on name of the current file above the editor.
Switch to a file by choosing it from the list.
(2) The Close File button allows you to close the current file.
(3) Use the Clear List button to remove all files from the list or tap on the Edit button to delete only some of the files from the list.