Once you have logged in to your subscription account you will see an option to "change date" beside your next order. Clicking this will allow you to change the next order date to whatever you like. Please note this will not change your order frequency, and that will remain the same unless you manually change it as well.
If you find you consistently need more or less of your order, you should consider changing the frequency of your orders.
Under 'Order Frequency' select 'Change frequency'. Select the new frequency you would like your product ordered.