How do I give permission to write images in my volume, I get this error

Warning: move_uploaded_file(/imagen/1002.jpg): Failed to open stream: Permission denied in /app/controllers/MatriculaController.php on line 391

Warning: move_uploaded_file(): Unable to move "/tmp/php4j8KCx" to "/imagen/1002.jpg" in /app/controllers/MatriculaController.php on line 391 Error al mover el archivo.}