>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
>

still very slow .
about 2,00,000 replacements takes a lot of time