Composer update Drupal core, 8.5.1 to 8.5.2 Nothing to Install
You can try to delete the vendor directory and the composer.lock file, and then run the update again.

UPDATE: 


You need to add the line "drupal/core": "~8.5.2" to the 'require' statement (or update it to whatever version you require).
Then remove the drupal/core line from the 'replace' statement.
Add the following line somewhere (at the end? I believe this should replace the existing repositories statement):

"repositories": { 
  "drupal": {
    "type": "composer",
    "url": "https://packages.drupal.org/8" 
  }
}


Then delete the vendor and composer.lock, and try again.

It should work.