Optimize MySQL update statement
Think about something like

UPDATE PCL, ( SELECT PCL.SrNo, MIN(HST.Date) minDate
              FROM PCL, HST 
              WHERE HST.SYM=PCL.INST 
                AND HST.DATE &gt;=  PCL.Date 
                AND HST.HP &gt;= PCL.T2 
                AND PCL.BS = 'B' 
                AND PCL.T1A IS NOT NULL
              GROUP BY PCL.SrNo ) subquery
SET PCL.T2A = subquery.minDate
WHERE PCL.SrNo = subquery.SrNo;


Of course, proper indices on both tables is safe.