Subscriptions can be managed through your online account. On the Account Overview tab, you'll see the dropdown menu "Subscription Settings" next to the heading Current Subscriptions. Here you can choose Delay delivery and choose a new date that suits you (this can be earlier or later than the current dispatch date).
If you need your refills ASAP, you can go onto Delay delivery then click the button "Get delivery now". Your refills will be processed the next day, and then be on their way to you shortly after.
Check out this video below which guides you through the process!