Update all values in a column with individual values
It should be this simple: 

UPDATE u
SET u.P60_Date = DATEADD(DAY, -2, u.P60_Date)
FROM dbo.US_P60 AS u


If you want to be sure, you can do:

SELECT *
INTO dbo.US_P60_bk
FROM dbo.US_P60


Then validate the update is what you want. If it is, drop the _bk table.