DATEDIFF combined with UPDATE STATEMENT

Question

I have a query below:

select DATEDIFF(DAY, DateRequested, GETDATE()) as NumberOfDays from tblSeriesRequest

if the NumberOfDays is greater than 30, then I want to perform an update

UPDATE [dbo].[tblSeriesRequest] SET [IsCancelled] = 1

I tried the query below but it updates the whole rows.

if exists(select 1 from tblSeriesRequest where (DATEDIFF(DAY, DateRequested, GETDATE())) > 30 and IsApproved <> 1)

    begin
        UPDATE [dbo].[tblSeriesRequest] SET [IsCancelled] = 1
    end

 

Can anyone show me how to do this? Many thanks in advance