Update Symfony components
As Ryan Weaver mentions,


  The current version of Drupal - 8.4 - requires Symfony 3.2.8 or
  higher, but less than 4.0. In practice, this probably means that your
  Drupal 8.4 project is using the latest Symfony 3.2.* version. But, you
  could technically upgrade your Symfony dependencies to version 3.4.
  
  Source: Symfony 4 and Drupal


Thus, you can, but only for minor versions. 



To quote Dries Buytaert,


  Our biggest dependency in Drupal 8 is Symfony 3, and according to
  Symfony's roadmap, Symfony 3 has an end-of-life date in November 2021.
  This means that after November 2021, security bugs in Symfony 3 will
  not get fixed. To keep your Drupal sites secure, Drupal must adopt
  Symfony 4 or Symfony 5 before Symfony 3 goes end-of-life. A major
  Symfony upgrade will require us to release Drupal 9 (we don't want to
  fork Symfony 3 and have to backport Symfony 4 or Symfony 5 bug fixes).
  This means we have to end-of-life Drupal 8 no later than November
  2021.
  
  Source: Drupal 7, 8 and 9


He says here that Drupal 8 will reach end of life of Nov 2021 because that's Symfonys 3's end of life. Thus, from this, one can conclude that you can't just upgrade D8 to Symfony 4 (another major Symfony version).