Assuming that the name of the table is "orders" and the date range for the past year is defined as the current date minus 365 days, the UPDATE statement would be:

```
UPDATE orders
SET OrderAmount = OrderAmount * 1.05
WHERE CustomerID IN (
  SELECT CustomerID
  FROM orders
  WHERE OrderDate >= DATEADD(day, -365, CURRENT_TIMESTAMP)
  GROUP BY CustomerID
  HAVING COUNT(OrderID) >= 5
);
```

This statement uses a subquery to identify the customers who have placed at least 5 orders in the past year, and then updates the OrderAmount for their orders by multiplying the existing OrderAmount by 1.05, which represents a 5% increase.
Data una tabella di ordini cliente con colonne OrderID, CustomerID, OrderDate e OrderAmount, scrivere un'istruzione UPDATE per aumentare l'importo dell'ordine del 5% per tutti gli ordini effettuati dai clienti che hanno effettuato almeno 5 ordini nell'ultimo anno.