Step-By-Step Guide:

1.  Ensure you are in Edit Mode
   
   
2. Select the document link that you wish to replace
   
   
3. Go to the Link tab and click Select File The File Library will open in a new window
   
   
4.  In the file info box on the right hand side:
   1. Click Edit file details
   2. Click the button next to Replace file
   3. Click Browse then select the new file from your computer
   4. Press Save
      
      
5. Click Select File (file library will automatically close) then click save

NOTE:  Ensure your file name does not start with a number.

Video Guide:

Click to see larger size