Students cannot delete or change files that have been submitted to the dropbox. The solution is to upload the file again with your changes. You can contact the instructor and they can delete the file for you as well.
If you have lost the original file, files in the dropbox can be accessed by clicking on the History button above the Dropbox folders list. Locate the file in question and save the file to your computer. The file can then be changed and uploaded again.
The original file will remain in the dropbox and the instructor will see more than one uploaded file so it is a good idea to inform the instructor by typing something in the Description field when you upload the file.