Go to your account and select 'Settings' > 'SIM settings'. At the next step, you'll need to order a multi SIM (which contains a nano SIM too).
Here, you can find a link to this page.
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).
Just ordered a new Mobile Vikings SIM card? No problem, the multi SIM card is the standard SIM card we send. It has all sizes, including the nano SIM card.