How to redirect URL with subfolder to the same URL but without subfolder?

Assuming you have already moved your site from the /blog subdirectory to the document root and changed the URL structure within WordPress itself, so the /blog subdirectory is not included in all your internal links, then you can either do a simple redirect (using mod_rewrite) in the /blog/.htaccess file (preferable):

RewriteEngine On
RewriteRule (.*) /$1 [R=302,L]

This will redirect a URL of the form /blog/foo to /foo, since the captured backreference ($1) does not contain the directory-prefix when used in a directory context.

Alternatively, if you want to delete the /blog subdirectory altogether then you can do something like the following at the top of the root .htaccess file, before the WordPress front-controller:

RewriteRule ^blog/(.*) /$1 [R=302,L]

Note that these are currently 302 temporary redirects. Only change to 301 permanent redirects when you are sure they are working OK.