Bug 4240 Can't edit class path entry (1GKEPIF)
There is no way to edit an existing entry for a library on the Java Build Path project properties page (other than editing the .classpath file directly). NOTES: EG (9/22/2001 11:42:11 AM) this is particularly annoying for variables. we should have an Edit button when a variable is selected.
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/wizards/buildpaths/SourceContainerWorkbookPage.java
if (cpe.getEntryKind() == IClasspathEntry.CPE_SOURCE) {
				
				
				if (!srcelements.remove(cpe)) {
					cpelements.remove(i);
					changeDone= true;
				} else if (lastSourceFolder == null) {
					lastSourceFolder= cpe;
				}
			}
		}
		if (!srcelements.isEmpty()) {
			int insertIndex= (lastSourceFolder == null) ? 0 : cpelements.indexOf(lastSourceFolder) + 1;
			cpelements.addAll(insertIndex, srcelements);
			changeDone= true;
		}
		if (changeDone) {
			fClassPathList.setElements(cpelements);
		}
	}
		
	private CPListElement createNewSourceContainer() {	
		IProject proj= fCurrJProject.getProject();
		String title= NewWizardMessages.getString("SourceContainerWorkbookPage.NewSourceFolderDialog.title"); 
		NewContainerDialog dialog= new NewContainerDialog(getShell(), title, proj, getExistingContainers());
		dialog.setMessage(NewWizardMessages.getFormattedString("SourceContainerWorkbookPage.NewSourceFolderDialog.description", fProjPath.toString())); 
		if (dialog.open() == dialog.OK) {
			IFolder folder= dialog.getFolder();
			return newCPSourceElement(folder);
		}