Can I change the bill date?

No, the billing cycle is always the same and is from the 26th of the previous month until the 25th of the current month.

For example, April's billing cycle closed on 25th April, which was then ready for payment from May 1st. The payment due date was then between 7th May. 

For Direct Debits, your payment was due on the 7th and taken from your bank account on the 7th-13th, depending on bank holidays/weekends.