code: public static FXManifest checkForUpdate() throws IOException {
          // We might be called even when FXLauncher wasn't used to start the application
          if (AbstractLauncher.manifest == null) return null;
          FXManifest manifest = FXManifest.load(URI.create(AbstractLauncher.manifest.uri + "/app.xml"));
          return manifest.equals(AbstractLauncher.manifest) ? null : manifest;
      }
docs: Check if a new version is available and return the manifest for the new version or null if no update.
  <p>
  Note that updates will only be detected if the application was actually launched with FXLauncher.
  
  @return The manifest for the new version if available