Hey there rob,
Hope you’re well! 🙂
First, I suggest you use a child theme http://codex.wordpress.org/Child_Themes or use css custom plugin like this: https://wordpress.org/plugins/simple-custom-css/ if you will customize your theme.
add this code to your child theme’s style.css or using the plugin mention above:
#primary {
float: right;
}
#secondary {
float: left;
}
Hope it helps! 🙂
Take care,
Calvin