Log into your account and head to the Plan page. You'll be able to see when your next order is due from there, and if you need to shift the date, just select "Edit Next Delivery."
One thing to bear in mind: if you've already got an order processing, any changes you make here will apply to the following delivery, not the one already on its way.
Need to change the date on an order that's already processing? Get in touch at feedme@differentdog.com or on 01952 456 850 and we'll do our best to help.