Since, hook_alter_schema() is removed from Drupal 8, how to alter any existing schema of some module in Drupal 8?

I have used `mymodule_update_8201` (`hook_update_N()`) function in my .install file

    use Drupal\Core\Database\Database;
    /**
     * Implements  hook_update_N()().
     */
    function mymodule_update_8201() {
     $spec = array(
        'type' => 'varchar',
        'description' => "New Col",
        'length' => 20,
        'not null' => FALSE,
      ); 
     $schema = Database::getConnection()->schema();
     $schema->addField('mytable', 'newcol', $spec);
     return $schema;
    }
But no new field or column gets added into my existing table. Is their any other way to achieve this?