Once an Expense has been paid, you no longer have the ability to make changes by double-clicking the expense (which re-populates the top section of the Expenses screen).
It is important however to maintain accurate accounts, so you may wish to inspect a previous payment to review some specifics, such as Account Codes used, horses/owners attributed, and more.
To review the details of a previous payment;
Go to Finance - Expenses
Set the Date to a previous expense date
Single-click (highlight) the payment in the lower portion of the screen
The details of the previous expense will be displayed in the upper-section of the Expense window, however the fields will not be editable.