Go to Cash Flow > Transactions. Select the icon to the right of the current category name and a list will pop up for you to choose from. You can select a default category or create your own.
To bulk edit you can change one category and then select the "create a rule" link that pops up in the bottom right corner.
Then, complete the form to edit all current and future transactions that match your set criteria. The system will automatically find all the transactions that match the new rule so you can easily apply to all of them at once.