without using premium plugins or something, what is the easy way ?
It depends slightly on whether you want to export the “main” site in the Multisite network or a different subsite. If the main site, it’s a bit simpler. Because the media uploads are probably already correct i.e. in the correct folders like /uploads/2024/
instead of /uploads/sites/3/...
or whatever.
If the media files are NOT correct, you should copy and paste the media uploads from the given subdirectory under /sites/
and into the normal directories as mentioned.
After that, remove any Multisite rules in your wp-config.php
After that, delete any tables in MySQL that are only for Multisite like:
wp_blogs
wp_blog_versions
wp_registration_log
wp_signups
wp_site
wp_sitemeta
wp_sitecategories (only, if exist)
See: https://wordpress.stackexchange.com/questions/30103/convert-a-multisite-wordpress-install-into-a-single-site