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
int nChosen= fileNames.length;
			
		IPath filterPath= new Path(dialog.getFilterPath());
		CPListElement[] elems= new CPListElement[nChosen];
		for (int i= 0; i < nChosen; i++) {
			IPath path= filterPath.append(fileNames[i]).makeAbsolute();	
			elems[i]= new CPListElement(fCurrJProject, IClasspathEntry.CPE_LIBRARY, path, null);
		}
		fDialogSettings.put(IUIConstants.DIALOGSTORE_LASTEXTJAR, filterPath.toOSString());
		
		return elems;
	}
	
	private CPListElement[] chooseVariableEntries() {
		NewVariableEntryDialog dialog= new NewVariableEntryDialog(getShell(), null);
		if (dialog.open() == dialog.OK) {
			List existingElements= fLibrariesList.getElements();
			
			IPath[] paths= dialog.getResult();
			ArrayList result= new ArrayList();
			for (int i = 0; i < paths.length; i++) {
				CPListElement elem= new CPListElement(fCurrJProject, IClasspathEntry.CPE_VARIABLE, paths[i], null);
				IPath resolvedPath= JavaCore.getResolvedVariablePath(paths[i]);
				elem.setIsMissing((resolvedPath == null) || !resolvedPath.toFile().exists());
				if (!existingElements.contains(elem)) {
					result.add(elem);
				}
			}
			return (CPListElement[]) result.toArray(new CPListElement[result.size()]);
		}