WordPress Multi-Site installations are useful only if you are managing more than 4,5 sites that are using the same theme and similar layout on all the sites. Personally, I don’t see the long-term benefit of using multi-site WordPress and the usual “excuse” of website owners is that is much easier for them to log in only once.
WP multi-site is known to have an incompatibility issues with many plugins and usually the speed performance is not quite good.
Migration from WordPress Multi-Site to Single Site (the easiest way)
You can easily migrate to single site by exporting all the content and media files. See the screenshot below.
After you exported the backup file, import it to your new WordPress (single site) installation that you created. Log in to the new WP install, click “Tools” > on the bottom where it says “WordPress” click “Run Importer” and upload/import the backup file. During the import process do not forget to select option to import attachment and users.
Import Theme Files
If you are planning to use the same theme files as on the multi-site installation, backup/import theme by using the FTP or a file manager that you can find within your hosting account. Zip theme files on a multi-site/download/upload/unzip in a single site theme directory (Root > WP-Content > Themes)
If the theme that you just uploaded is not visible on the frontend, activate it in admin dashboard. (Appearance > Themes)
Media Library Files Location on WordPress Multi-Site
In case that you have to manually backup media files from your subsite, you can find all the files in this directory: Multi-Site Root directory > WP-Content > Uploads > Sites > 2. Number #2 is an ID of your subsite. If you have 3 websites in your Multi-Site network, the main website do not have an ID, the second site (subsite) you created has #2 ID, third one #3 etc…