change menu links for custom menu


there seems to be a custom menu in the header.
I say "custom" because there seems to be no menu under "Appearance/menu" in wordpress admin remotely resembling this.

There also seems to be no plugin applying this.

So under "Appearance/editor" in wordpress i looked through index.php header.php etc. etc. and still cannot find where and how to change the links.

change required because the "registration" link is linking to a non existing page and therefore giving a 404 error.

Can anyone guide me accordingly please ?
any kind assistance in this would be greatly appreciated.

kind regards,