How to change the 3 bar menu ?

Hello,

My english is not perfect but I’ll try to do my best to be understood ??

I’am using WordPress + Customizr with a child theme.
There’s a 3 bar menu for all devices (computer + mobile).
I would like to change this menu in order to have :
– on computer : an inline menu with texts (home, subject 1, subject 2…)
– on mobile : a button with the text “menu” and when we click on “menu” we can see “home, subject 1, subject 2…)

I think I have to put a code in style.css or functions.php but I really don’t know which one. Can someone help me ?

Thanks