It depends on what you scheduled and where. Here are the most common situations:
You scheduled a one-time portal payment but meant to set up recurring (autopay). No problem — your one-time payment will still go through with Split Pay, so there's nothing to undo. For future months, set up the recurring or autopay option in your portal with your Split Pay account and routing numbers as the payment method, so you don't have to schedule it each time.
You scheduled a portal payment using the wrong payment method (not Split Pay). If it hasn't processed yet, cancel it in your portal and reschedule using your Split Pay account and routing numbers. If it already processed, it was paid by your other method and Split Pay didn't cover that one — update your default payment method in the portal so Split Pay is used next time.
You meant to pay through your portal but set up a direct payment in the Split Pay app. Portal and Direct are two different Split Pay products, so a single payment can't be switched from one to the other. To pay through your portal instead, you'll need to cancel your current plan and reapply for Split Pay with the Portal product selected. Get in touch and we'll help you make the switch.
You scheduled a Split Pay direct payment for the wrong amount or date. In some cases you can still change it — see Can I cancel a scheduled direct payment to my landlord?.
If you can tell us exactly what you scheduled and what you meant to schedule, we can walk through the fix step by step.