You can’t change or edit the default categories which have been created. You can, however, create your own sub-categories.
Create a Category
To do this, select Change Category on the transaction your wish to recategorise. Select the main category that you would like for your custom sub-category to fall under. At the bottom of the sub-categories list will be a blank field for you to enter the name of your new custom sub-category, then select the + to confirm.
Delete a Category
To delete a category you have created, head to Settings, select Preferences and then Manage custom categories. Select the bin icon to the right of the category you wish to delete.