You can cancel a payment the same way that you would delete any other expense. Just tap/click on the payment in your list of transactions, and then click the "Delete" button. The payment will be removed, and your balances will update accordingly, as if you had never made the payment in the first place.
If you settled up via PayPal or Venmo, you'll also need to log in to your account on that service and cancel the money transfer. Unfortunately, Splitwise cannot directly cancel a money transfer once it has been initiated.