Using DateDiff function in UPDATE query in SQL Server

Dear All,

I have one table where I have 2 columns i.e. date and counter. I have created the stored procedure to get the counter from the request date to today’s date

create procedure testprocedure AS

select *,DATEDIFF (dd, date,GETDATE() ) as Counter from testtable

go

I want to update the counter column after getting the counter from the select statement. Any suggestion.

Thanks