Please go to Upgrade page and select any package on the website:
On the upcoming webpage, please click on the Add additional billing data button:
The following section will pop up where you can add or change your billing information:
You can choose between the individual option if you want to buy premium memberships as a normal customer or the company option if you want to make a purchase as a firm. Please be aware that you have to state a legitimate VAT ID for the second option.
Afterwards, you can just leave the page with the changes as it is. Please be aware that all the changes will only apply to the billing of all upcoming purchases and renewals and not to the billing of purchases you have already made.
To change your payment method or payment details, please refer to the following article.