How do I change Bootstrap 3 column order on mobile layout?

You cannot change the order of columns in smaller screens but you can do that in large screens.

So change the order of your columns.

<!--Main Content-->
<div class="col-lg-9 col-lg-push-3">
</div>

<!--Sidebar-->
<div class="col-lg-3 col-lg-pull-9">
</div>

By default this displays the main content first.

So in mobile main content is displayed first.

By using col-lg-push and col-lg-pull we can reorder the columns in large screens and display sidebar on the left and main content on the right.