I found a way which maybe is useful for anybody.

1. As I want to change the deleted field, restrictions have to be removed:

2. To handle complicated SQL situations the add() function is appropriate.

Thus, this code works:

$queryBuilder = $this->connectionPool->getQueryBuilderForTable(tx_myevents_domain_model_event);
$queryBuilder
    ->getRestrictions()
    ->removeAll();
$queryBuilder
    ->update(tx_myevents_domain_model_event)
    ->add( 'where', '(datetime_end + (days_until_deletion * 86400)) < UNIX_TIMESTAMP()')
    ->set('deleted', '1');