If you have a blog at http://blog.example.com/ and you want to move it to WordPress at – http://example.com/
You can put following htaccess rewrite rules on blog.example.com
RewriteEngine On RewriteRule ^(.*)$ http://example.com/$1 [R=301,L]
If you want blog homepage to redirect to – http://example.com/blog/ (you will need this if http://example.com/ has a static-homepage) then you can try following:
RewriteEngine On RewriteRule ^/?$ http://example.com/blog/ [R=301,L] RewriteRule ^(.*)$ http://example.com/$1 [R=301,L]
For other way round, refer to moving a WordPress blog from subdirectory to subdomain.