Click on 'File Manager' located on the left hand side of the screen and then press on the one labelled 'Documents'.
You should now see a list of all the Documents you have on the system. Click on the name of the one you wish to delete - if you can't find it there in the list you can use the grids to search for it (fear not, your documents haven't been removed or disappeared, just clear the grids).
Then all you have to do is press the Delete button.
You will then receive the confirmation warning and if you are happy to delete all you have to do is press OK.