How to change the location of the menu

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I had a look at the GeneratePress theme for you; adding this custom CSS should do it:

.nav-float-right .inside-header .main-navigation {
    float: none !important;
}

EDIT: Or, as Madalin showed, a non-!important version:

html .nav-float-right .inside-header .main-navigation {
    float: none;
}