Locate the Expense that you would like to delete.
Click on the Delete icon in the page actions menu.
You will be prompted to confirm this action. It cannot be undone!
You can't delete an Expense if a payment has been made to it. If you really need to delete the Expense even if a payment as been made to it (and generally speaking there needs be a pretty good reason to do this)...then you will have to delete the payment first. This may not be possible if you have already reconciled this payment. In such a situation you will then also have to uncheck this payment in the Bank Reconciliation page (in other words unreconcile this payment) before deleting the payment.