The first step is to log-in to your account.
Once logged in, navigate to the section 'Orders and Replenishments' from within your account. Next, simply click on the link 'Cancel Replenishment', and confirm your choice.
Please be aware - if your order is canceled, any progress made towards your free 6th item will be lost. Instead, you may want to pause your order, ready to reinstate it at a later date.