I would like to clarify that for heavy customization, you should hire a developer to carry out such work, customization support is out of JA support scope.
If you are a developer you can change it from templates/ja_decor/tpls/blocks/header.php file and change the mega menu to new line
<div class="col-xs-12 <?php echo $mainnavsize ?> pull-right">
<div class="t3-navbar navbar-collapse collapse">
<jdoc:include type="<?php echo $this->getParam('navigation_type', 'megamenu') ?>" name="<?php echo $this->getParam('mm_type', 'mainmenu') ?>" />
</div>
</div>
and custom the css style