multi sites => one site

If you don’t have much content in the sites, I think I would just start over with a new install of WP as a single site.

If you have content in subsites, you will need to export that content so that you can import it to the new single site.
http://www.wpbeginner.com/wp-tutorials/how-to-move-a-site-from-wordpress-multisite-to-single-install/

If you don’t have any content on the subsites — only the main site — you can just work backwards from the multisite install directions, undoing the setup.