T-SQL 2005 Update Triggers

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T-SQL

I have a 3rd party software package with a MS SQL Server 2005 database. One of the tables (ADDRESS) has a column MODIFYDATE which is a DateTime that should be updated each time the record is changed, but due to a bug in the software, it is not being updated.

My solution is create an UPDATE TRIGGER on the table, that buts the current Date and Time into the field.

CREATE TRIGGER trgUpdateModifyDate
   ON  sysdba.[ADDRESS]
   FOR UPDATE
AS
BEGIN
    SET NOCOUNT ON;
    UPDATE [ADDRESS] SET MODIFYDATE = GETDATE() WHERE ?????
END
GO

My question is, how do I identify which row(s) have been changed?

Thanks