Here's one way, I think
update it
set Add1 = s.Add1
,Add2 = s.Add2
from dbo.items_test it
join dbo.Staging1 s on it.Location = s.Location
and it.ProdCode = s.ProdCode
and it.Customer = s.Customer
where it.Date_Effective = (
select max(date_effective)
from dbo.Items_TEST t
where t.Location = it.Location
and t.ProdCode = it.ProdCode
and t.Customer = it.Customer
)
If you haven't even tried to resolve your issue, please don't expect the hard-working volunteers here to waste their time providing links to answers which you could easily have found yourself.