Flaticon subscriptions are linked to a specific payment method. This means that, if your Premium plan has been processed via Paypal, you cannot change to credit/debit card method within the same subscription and the other way around.
In any of these cases, you will have to cancel the automatic renewal of your subscription. Then, once your current plan ends, you can purchase a new Premium plan using the new payment details.
If you have paid by credit/debit card and want to update it to a new card number, this can be done from the tab “My subscriptions” on your Premium account, by selecting the “Modify billing information” option, as shown below.