In the Manage Payment screen there is no 'Edit' button. This is to provide a proper 'audit trail' of payments recorded in Simple Invoices.
If you've made a mistake with a payment entry the best option is to reverse the entry and entery it again correctly
Reverse the entry - what the?
This basically mean just doing a negative entry for the same amount as the original entry.
ie.
If your entered $110
but you should of entered $1100
to reverse this entry enter -$110 against the same invoice and then enter the correct amount of $1100