Transfer a Child Theme to a new WordPress Site

Install old Theme twentyfifteen again

Create Child Theme twentyfifteen-child

• Create directory twentyfifteen-child and
• Transfer the following files via ftp client from your old Child Installation
  • Upload : functions.php
  • Upload styles.css

Activate Child Theme twentyfifteen-child

Adding Menu

• For menu top level entries use categories
• Categories can be created when saving a new Page

Reference

• WordPress Child Theme erstellen – ultimative Anleitung 2021 (webtimiser.de)