You added this on line 2029 in your css –
.nav-primary {
max-width: 750px;
}
That’s where the width limit is coming from. Remove that altogether first.
Then you’ll want to replace float: left;
on line 2012 with display: inline-block;
in order to center the entire nav –
.genesis-nav-menu .menu-item {
display: inline-block;
/* float: left; */
list-style-type: none;
margin: 0;
padding: 0;
text-align: left;
}