UPDATE sf

set JUL_SAL = isnull(sf.JUL_SAL,0) + isnull(ss.SLS_DOL,0)

FROM

DIAPLUSDB.dbo.SalesForecast sf

left join (

SELECT CSCODE, PLT_NO, SUM(SLS_DOL) AS Total_SLS_DOL

FROM SALES_SUMMARY

WHERE PPPERIOD = '201207' and SLS_DOL is not null

) ss on ss.CSCODE = sf.CustID and ss.PLT_NO = sf.PlantID

I'm not sure why do you need LEFT JOIN here.

If there are not corresponding records in SALES_SUMMARY the query will add 0 to JUL_SAL, leaving the value in there unchanged, but adding an overhead of a useless update.

Considering you WHERE clause effectively forced INNER JOIN anyway, I'd made it explicit.