Your personal discount is valid for one year. You can't extend it.
Your personal discount is waiting for you at the checkout. Click on "I'm going to order" in your shopping cart and log in with your My Coolblue account. Your discount is available under "Payment method". Do you want to save your discount? Simply uncheck it. The discount is valid for one year.
Don't have a Coolblue account yet? You can create an account via the email containing your personal discount. The personal discount will be ready for you at the checkout.