Hi,
It's pretty easy - in the Related Documents section, you just need to right click on the document and select Delete. One thing to watch out for - make sure you're not editing the item when you right-click on it (you can get into edit mode sometimes accidentally by clicking on an item already selected).
If the document is located in the same folder as your project, you'll be given an option to delete the document in addition to removing the reference (otherwise, it leaves the document alone and just removes the reference).
Hope this helps, but let me know if you have any other questions,
Doug