UPDATE T1 SET
Opening_date = T2.Opening_date,
ORDINAL = DATEDIFF(DAY, T2.Opening_Date, T1.DATE)
FROM FactTransaction T1
INNER JOIN DimStore T2 ON T1.cod_Store = T2.Cod_StoreKey;
...if I understand you correctly.
Edit: Ok, just go with:
UPDATE T1 SET
ORDINAL = DATEDIFF(DAY, T2.Opening_Date, T1.DATE)
FROM FactTransaction T1
INNER JOIN DimStore T2 ON T1.cod_Store = T2.Cod_StoreKey;