UPDATE with JOIN: Incorrect syntax near the keyword 'RIGHT'
Try something like this:

UPDATE viewOrderAlla
SET viewOrderAlla.tblOrder_verkligtid = 0
FROM tblTid
RIGHT JOIN viewOrderAlla ON tblTid.sOrdernr = viewOrderAlla.tblOrder_ordernr
WHERE viewOrderAlla.tblOrder_verkligtid &lt;&gt; 0
    AND tblTid.iTidID IS NULL
    AND viewOrderAlla.tblOrder_levdatum &gt;= DATEADD(month, - 6, GETDATE())