update table1 as t1,table2 as t2 set t1.field1=t2.field2 where t1.uid=t2.uid;

$queryBuilder = GeneralUtility::makeInstance(ConnectionPool::class)
    ->getQueryBuilderForTable('table1');
$queryBuilder->getRestrictions()
    ->removeAll();
$queryBuilder->update('table1')
   ->join('table1', 'table2', 't2',
   $queryBuilder->expr()->eq(
       't2.uid',
       $queryBuilder->quoteIdentifier('table1.uid')))
   ->set(
       'table1.field1',
       $queryBuilder->quoteIdentifier(
           't2.field2'),
       false
   )->execute();