From my.pogoplug.com, simply place your cursor on the desired file or folder. The "abc|" icon will appear right next the file/folder name; click the icon and you will be placed in edit mode. Type in a new name, then click anywhere outside of the edit area.
Don’t forget to maintain the file extension. E.g: If you are renaming a photo (.jpg), make sure the new file name still has .jpg at the end.