Go to your account and select 'Settings' > 'SIM Settings'. At the next step, you'll need to order a multi SIM (which contains a micro SIM too).
A € 5 administrative cost is charged for this swap, which will be deducted from your calling credit. Make sure that, when requesting a new SIM card, you have enough calling credit on your account. When you receive the new SIM card, remember to activate it via your account (on www.mobilevikings.be/activate).