Hi annabelclayton
You can change your direct debit details and/or date online via My Virgin Media.
You can change your preferred payment date by signing into your My Virgin Media account, click into the View bill section, and from here you can check your Direct Debit details and change your payment date.
Changing your Direct Debit date will alter your next payment amount, to bring you in line with your new billing cycle. It'll be shown in the Package charges section of your bill. The rest of your payments will then be the usual amount.
*** There is a restriction of only allowing you to change your payment date twice in any 12 month period, also on the dates you can make the changes. For example if I were to try and change my direct date today then I would be unable to do so as my billing date is the 23rd December and my payment date is 9th January. I would need to wait to make those changes between between 9 January and 23 January (the dates in between the actual payment date and next billing date)