To edit existing transaction categories, first go to MoneyWiz Settings by clicking on the gear icon in the menu. Once you're in Settings, go to Categories section.
Click on the category you wish to edit. If you need to edit a sub-category, please click on the little triangle to the left to expand the sub-category list to be able to view it.
Remember that if you wish to change the category colors, you need to first ensure that colorful categories are enabled in Transaction layout settings.