1. Add a DNS record:
In your DNS zone editor, you must add an entry to map the new subdomain(s) to the web server. In Cpanel, you would need to go to the subdomain section. Add a subdomain like mysubdomain.mydomain.com and link it to the subfolder of your web server where your WordPress multisite is installed.
2. Change wp-config.php
define('SUBDOMAIN_INSTALL', false); => define('SUBDOMAIN_INSTALL', true);
3. Add redirects to htaccess
Add the following redirects at the top of your htaccess file to redirect from your former subdirectories to your new subdomain URLs:
RewriteRule ^formersubdirectory/(.)$ https://newsubdomain.tomydna.com/$1 [R=301,L]
Check if your redirection directives are OK in htaccess.
4. Change your site URL in WordPress
Login to your main network site. Choose Network admin > site. Choose the website that you want to modify and click edit. Change the “site address URL” from the subdirectory to the subdomain URL
