Howello WP theme contains a button in the header

To change its text or its link, please go to themeoptions/Layouts.

If you want to change its background or hover color, Please enter below css in wp-admin/appearance/customise/additionalCSS box.

.header_menu .navbar.navbar-default .nav.navbar-nav.navbar-right .book_btn .book_now_btn {
    background: #000000;
    color: #FFFFFF;
    border: 2px solid #000000;
}
.header_menu .navbar.navbar-default .nav.navbar-nav.navbar-right .book_btn .book_now_btn:hover {
    color: #000000;
    background: 0 0;
}