Change main site

I have a multisite. I have a top level domain that is my main site.

I’ll call it maindomain.com

Then I have site 3 which is u.maindomain.com and I have a url applied to that and we’ll call it seconddomain.com

I then made a site 4 that was a copy of maindomain.com 2013.maindomain.com so that I could redesign the site. I have re-designed the site and now I want to make that the maindomain.com

How can I make 2013.maindomain.com the maindomain.com site?