Create the subdomain in the DNS records
Please read all the actions before starting.
DNS records
In the hosting administration, you should add an A record to map the new subdomain to the IP address of your web server. For

Map the subdomain to the multisite install folder
Your subdomain should point to the folder where the WordPress multisite is installed. For example, if you use Cpanel, go to “subdomain”, add your subdomain and choose the folder of your WordPress multisite folder that already exists. This job will also create a DNS record automatically.
Change the .htaccess file to the following
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L] # add a trailing slash to /wp-admin
RewriteRule ^wp-admin$ wp-admin/ [R=301,L]
RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^(wp-(content|admin|includes).*) $1 [L]
RewriteRule ^(.*\.php)$ $1 [L]
RewriteRule . index.php [L]
Change the wp-config:
define(‘WP_DEBUG’, false);
define(‘MULTISITE’, true);
define(‘SUBDOMAIN_INSTALL’, true);
define(‘DOMAIN_CURRENT_SITE’, ‘yourdomain.com’);
define(‘PATH_CURRENT_SITE’, ‘/’);
define(‘SITE_ID_CURRENT_SITE’, 1);
define(‘BLOG_ID_CURRENT_SITE’, 1);
Change the name of the domain in wp
In WordPress, login into your network site. Go to “sites” and change the urls of your websites from the subfolders “yourdomain.com/subfolder/” to your subdomain “subfolder.yourdomain.com”
Change the links on your webpages
Use a search and replace plugin to change the URL links on your web pages from yourdomain.tld/yoursubfolder/example to subfolder.yourdomain.tld/example
You can do it using a plugin that will search and replace the strings in bold above in the table wp_posts which contains your wordpress pages and posts.