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To edit an existing Categories, you must go to the appropriate app.
How to edit an existing Categories
To edit a Categories that appears in a Contact Form, Event Creation Form, Event Guest Booking Form, or Survey, go to the Categories App - Edit a Categories field.
To change which Categories is used as the set of answer choicesin a Multiple Choice Survey Question, go to the Goals App and follow the steps to edit a Multiple Choice Tracker. But if you want to simply edit an existing list, go via the Categories App (steps described below)