Order blog post by "Last Updated" and not "Published Date"

I display the "Last Updated" date in my posts rather than "Published Date", however, the order of my blog posts goes by the latter. How do I bump posts to the top of the blog when I update the post without editing the publish date?

Is this something I can select in the GP theme, so do I need to edit my CSS?

I have the following in CSS:

.posted-on .updated {
display: block;
}
.posted-on .published {
display: none;
}
.posted-on .updated:before {
content: "Last updated: ";
}