If you need to edit a memorized transaction, you can do so. If you are making significant changes, then it might be worth simply deleting the memorized transaction and re-entering it. However, editing the memorized transaction is easy, so both options are fine. To edit a memorized transaction, simply:
Open the Memorized Transactions List from the Lists menu.
Open the memorized transaction you want to edit by double clicking it.
Then click Replace in the Replace Memorized Transaction window.
Assuming you are simply editing the memorized transaction and do not wish to actually enter it at this time, close the transaction window without saving.