Highlight the line and click the Reverse button.

This will bring up a warning box. If you are sure you want to reverse this entry, click OK. 

Enter a Reversal Reason

Click Save to action the Reversal. 

If you don't want to reverse the transaction, click Cancel to exit this screen. 

(Unsure if this is still the case) If the period in which the incorrect Express Payment Entry was entered has been closed, it must be corrected by Journal entry. See Processing Journal Entries for information on how to complete this. The journal will take the following form:
DR: Bank account the expense was paid from for the full amount of the expense CR: Expense account entry originally expensed to for the amount net of GST, ensure tax category is 'Not Applicable' 
CR: GST Tax Paid for any GST, ensure tax category is 'Not Applicable'

If the period in which the incorrect Expense Entry was entered has been closed, it must be corrected by processing a negative Expense Entry. The original entry must be reversed exactly with a minus sign '-' in front of the Amount Payable and the amount in the allocation grid.