Start by logging in to your account - you may be able to schedule or reschedule your pick up there by clicking the 'Choose pickup window' button and selecting your preferred pickup window.
If you don't see the option to 'Choose pickup window', you can contact your carrier directly to schedule or reschedule your pick up. You can find their contact information in your account, in an SMS (if you've opted in), and also in an email we've sent you.