Since, hook_schema_alter() 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;
    
But no new field or column gets added into my existing table. Is their any other way to achieve this? 

@edit

module `a`

`a.install`

    function a_schema() {
      $schema['my_table'] = array(
        'fields' => array(
           'type' => array(
              'description' => 'registration type',
              'type' => 'varchar',
              'length' => 255,
            ),
         ),
      );
    }

module `b`

    /**
     * Implements  hook_update_N()().
     */
        function b_update_8201() {
         $spec = array(
            'type' => 'varchar',
            'description' => "New Col",
            'length' => 20,
            'not null' => FALSE,
          ); 
         $schema = Database::getConnection()->schema();
         $schema->addField('my_table', 'newcol', $spec);
         return $schema;
        }