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Never edit the parent theme unless you copy the files out of it and create your own. By editing the parent theme directly you’re risking it not working correctly in future versions.

Copy the header.php out of the parent theme folder, and into your current active theme folder. bp-default, or whatever else you might be using.

WordPress will automatically override the parent header.php because files in child themes have a higher priority over parent themes. You won’t need to do anything additional to tell it what to do.