change navigation menu orientation

Hi everyone, I am would like to change the orientation of the elements of my navigation menu, in the header. I mean instead of having:

“Home” -> “Contact” -> “Category1” -> “Category2”

I want :

“Home”
“Contanct”
“Category1”
“Category2”.

I guess that in order to do that, I need to change my header’s html, but how can I exactly do that? What would the code look like and where should I interfere ?

Thanks in advance