It depends on indexes. You could use this code. You did not specified if symbol + series + date generate a unique key for records in each table.

set talk off
use bhav_data
use del in 2
sele bhav_Data
scan all
   sele del
   locate for symbol=bhav_data.symbol and ;
        series=bhav_data.series and date=bhav_data.date
   if found()
        replace delqty with delqty in bhav_data
   endif
endscan
close data