Hi does anyone know how to update a datepart?

update table set datepart(dd, tablename) = '30'
where ??

What I want to do is udpate records date data but only the day
so where all of the records > '2009-07-25 00:00:00.000'
are updated to 30 i.e. 2009-07-30, I want to leave the timestamp portion alone in the records.

Thanks in advance.
Sonia

What happens if the date is '2009-08-01', does it go back to 7/30 or does it go to 8/30?