Copy subsite to main site

I’m using multisite on my wordpress installation. The primary site ended up not being the main site for everything I wanted to do. I am wanting to move the content from one of my subsites to the main site to shorten the url.

Most of my searches on this matter deal with copying the main site to the subsite, which I don’t want! I can’t delete the main site because it is the primary site, therefore, I want to move a subsite into the primary site.

How do I do this?