Of course you can. Right here. The only thing to keep in mind is to make sure you have no outstanding balance in your subscription. As soon as your balance has been settled, you can switch.
Per month there's a limit to the amount of times you can switch between a subscription and a prepaid SIM card.
Here's some examples:
- Subscription -> prepaid SIM card -> subscription. After this you can no longer switch to a prepaid SIM card in that same month.
- Prepaid SIM card -> subscription -> prepaid SIM card. After this you can no longer switch to a prepaid SIM card in that same month.
Also check: "Can I change the price plan for a subscription?"