How can I upgrade to core 9?
No, you can't. drush pm-update has been deprecated for a while. The command is still available in the Drush 8.x branch but in later revisions it only exists as a signpost to inform the user how to move to Composer:


  The pm-updatecode command was removed. Please see composer update --dry-run and composer update.


Noting that Drush 8 isn't supporting revisions past 8.3.x or 9.x, you should look into upgrading Drush and moving to a Composer-driven upgrade process.