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/NewVariableEntryDialog.java
for (int i= 0; i < selected.length; i++) {
				IPath filePath= new Path(((File) selected[i]).getPath());
				IPath resPath=  new Path(elem.getName());
				for (int k= elem.getPath().segmentCount(); k < filePath.segmentCount(); k++) {
					resPath= resPath.append(filePath.segment(k));
				}
				paths[i]= resPath;
			}
			return paths;
		}
		return null;
	}
	/*
	 * @see Dialog#buttonPressed(int)
	 */
	protected void buttonPressed(int buttonId) {
		if (buttonId ==  IDialogConstants.CLIENT_ID) {
			List selected= fVariableBlock.getSelectedElements();
			if (selected.size() == 1) {
				IPath[] extendedPaths= chooseExtensions((CPVariableElement) selected.get(0));
				if (extendedPaths != null) {
					fResultPaths= extendedPaths;
					super.buttonPressed(IDialogConstants.OK_ID);
				}
			}
		} else {
			super.buttonPressed(buttonId);
		}
	}
}