Quest Diagnostics: https://appointment.questdiagnostics.com/as-home

You should have an option to cancel/reschedule the appointment in the email that you received when scheduling the appointment, but if not you can do it on Quest Diagnostics website.

LabCorp: https://www.labcorp.com/labs-and-appointments

Same as Quest, you should be able to cancel/reschedule from the email or find instructions to reschedule or cancel on the confirmation you received. Once on the LabCorp page, this box will be located to the bottom right of the page. This is also the same way to schedule an appointment.