Contact customer services and ask them to make the change when ringing 150 (from a VM phone) or 0345 454 1111 (from any phone) go with the Billing option once into the main menu, then go with one of the sub-billing options and make the request. It will come into effect for the next bill, not the current.
Or ask via the Webchat using a laptop or desktop, between 8AM to 8PM M-F and 9AM to 5PM Sa: Account and Billing -> Understanding my Broadband, TV and Home phone bill -> Chat online. If it says the agents are busy, reload the page every three minutes and check again repeatedly, until you get through to an agent.
______ Any opinions expressed by myself are entirely my own and do not represent Virgin Media in any way.