In the Web Portal > CRM > Payments > select the payment you wish to remove > scroll to the bottom of the page > Select Action > Undo Payment > Go.
or
In the Web Portal > CRM > Invoice > Edit > select the hyperlink payment to remove it > select the payment you wish to remove > scroll to the bottom of the page > Select Action > Undo Payment > Go.
In the App > Accounts > Invoices > Edit > Payments > swipe to the left to remove the payment.
(Video shows adding the payment then removing it)