You can edit your transactions from the Transaction page on web and mobile. The merchant name, the category and the date are all editable. Expand the transaction to see all the details.
To edit multiple transactions at a time, you can create a Transaction Rule. For instance, if you'd like to re-categorize all your Nordstrom transactions from Shopping to Clothing you can do so from the Customize Rules page. When you edit a transaction, you will get a popup that invites you to create your action as a custom rule.
To hide a transaction, expand to the details and toggle the Hide option. Hiding a transaction will remove it from your Budget calculations and the Spending trends on the Cash Flow page. It will continue to appear in your Transaction list and in the Transaction search.