Hello! After installing and importing sample posts our boss wants to change URL of our posts. I’ve already changed site.com/blog to site.com/news but posts remain the same URL: site.com/blog/post-name instead of site.com/news/post-name.
Sorry but this is not related to theme question. It seems you have registered the main domain name as site.com/blog. If you made the changes of domain correctly then perhaps clearing cache of wordpress and server will help reflect the changes sooner.
Or check your database for in case there still is present the old url in any records.
Contact your host for more regarding it.