To change your service's IP address, navigate to your service's control panel by clicking "Services", "My Services", and then the service that needs the IP change. When you have arrived at the control panel, click "Modify Service" and then "Switch IP". You will be presented with 2 options, regular and custom. Both options will randomly assign you a new IP address after payment has been made, but the custom option will additionally allow you to specifiy requirements for the new IP address if the randomly assigned one is not suitable for you.
After your request has been submitted, a ticket and an invoice will be generated for you to pay. To make your payment, go to "Billing", "My Invoices", and pay the invoice referenced in the generated ticket.