Here is the UPDATE statement you can use:

```
UPDATE customer_orders
SET OrderAmount = 0
WHERE OrderDate <= DATEADD(month, -6, GETDATE()) AND OrderAmount < 100;
```

This statement will update the `OrderAmount` column to 0 for all customer orders that were placed more than 6 months ago and had an `OrderAmount` of less than $100. The `DATEADD` function is used to subtract 6 months from the current date, and the `WHERE` clause filters the results to only include orders that match the conditions.
Data una tabella di ordini cliente con colonne OrderID, CustomerID, OrderDate e OrderAmount, scrivi un'istruzione UPDATE per impostare l'importo dell'ordine su 0 per tutti gli ordini effettuati più di 6 mesi fa e con un importo dell'ordine inferiore a $ 100.