Depending on whether you have completed or saved the Expense to Drafts will depend on where to go to re-open it.
Go to Finances > Invoices, Expenses and Deposits
Once you have opened Invoices, Expenses and Deposits you can edit the Expense. Open Awaiting Payment or Draft and select on the Number this will open in the Edit mode where you can choose to Add Payment, Edit or Delete.
If an Expense has already been processed you will not be able to Edit it - instead you will need to create a new Expense with different details. You can however Delete by going to Transactions > Action > View Expense. This will open up the Expense where you can Delete.