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/LibrariesWorkbookPage.java
public LibrariesWorkbookPage(IWorkspaceRoot root, ListDialogField classPathList) {
		fClassPathList= classPathList;
		fWorkspaceRoot= root;
		fSWTControl= null;
		
		fDialogSettings= JavaPlugin.getDefault().getDialogSettings();
		
		String[] buttonLabels= new String[] { 
			 NewWizardMessages.getString("LibrariesWorkbookPage.libraries.addjar.button"),	
			 NewWizardMessages.getString("LibrariesWorkbookPage.libraries.addextjar.button"), 
			 NewWizardMessages.getString("LibrariesWorkbookPage.libraries.addvariable.button"), 
			 NewWizardMessages.getString("LibrariesWorkbookPage.libraries.advanced.button"), 
			 null,  
			 NewWizardMessages.getString("LibrariesWorkbookPage.libraries.setsource.button"), 
			 null,  
			 NewWizardMessages.getString("LibrariesWorkbookPage.libraries.remove.button") 
		};		
				
		LibrariesAdapter adapter= new LibrariesAdapter();
				
		fLibrariesList= new ListDialogField(adapter, buttonLabels, new CPListLabelProvider());
		fLibrariesList.setDialogFieldListener(adapter);
		fLibrariesList.setLabelText(NewWizardMessages.getString("LibrariesWorkbookPage.libraries.label")); 
		fLibrariesList.setRemoveButtonIndex(IDX_REMOVE); 
	
		fLibrariesList.enableButton(IDX_EDIT, false);
		
		fLibrariesList.setViewerSorter(new CPListElementSorter());
	}