BuddyPress.org

Helene
A child theme lives in its own directory, and inherits any files not presenting the child theme, from its parent theme. So what you would do is create a child theme of BP Default and copy into your child theme, BP Default’s header.php (for example – what you want may be on another page) and customise it. You’ll only have to worry about updating it in the future if BP Default itself is updated (probably not for a couple of months, at least).