Create a special menu for small screen devices

Hello,
I notice that below 768px, the menu becomes a dropdown.
I have 2 menus (primary and secondary).
I would like to gather both menus in a third (mobile-menu).
But I don’t know how to put this mobile menu instead of primary menu in the dropdown.
Does somebody could help me?
Cheers.

Hello,
I up this topic. Could the author or someboy else help me on this.
Regards