Move a file in drupal 8

file_move(FileInterface $source, $destination = NULL, $replace = FILE_EXISTS_RENAME)

Example:

$file = \Drupal\file\Entity\File::load($fid);

file_move(FileInterface $source, 'path/to/destination');

https://api.drupal.org/api/drupal/core!modules!file!file.module/function/file_move/8