To change DD details you can do it online through My VM by signing in with your primary My VM account (the one that can see the bills etc.) Then go into My Bills -> Change Direct Debit. The change would come into effect for next month.
I'm not sure where the option is if you have a mobile account, but the option is in there in the online account and it too likely only comes into affect for the next month.
To change your direct debit setup via a phone call (for broadband/TV/phone) ring 150 from a VM phone or 0345 454 1111 from any phone. -> Option 1 -> Confirm your details -> Option 1 (Billing) -> choose the direct debit option.
For the mobile side, ring 789 from the mobile in question or 0345 600 0789 from any phone.
______ Any opinions expressed by myself are entirely my own and do not represent Virgin Media in any way.