UberMenu Integrates with TwentyTwelve, but there is residual styling (see also: residual styling troubleshooting guide) from the theme.

The best practice solution is to create a child theme, copy the header.php to the child theme, and then:

1. Remove the residual styling by removing the main-navigation class from the nav element

2. Remove the Toggle Button markup altogether

<h3 class="menu-toggle">Menu</h3>

If you do not wish to create a child theme, you can use Custom CSS – though this is not a best practice solution.

You can hide the toggle with CSS

.menu-toggle{
  display:none !important;	
}

and remove some of the residual styling with

#megaMenu ul.megaMenu li.menu-item > a,
#megaMenu ul.megaMenu li.menu-item > span.um-anchoremulator{
    text-transform:none;   
}