1. by clicking on 'edit' at the bottom of the moot - this is the button on the right hand side of the bar where categories of the moot are.
2. by clicking on 'edit' in the 'more actions' panel - if you roll over on judgements icons on the right hand side of the moot you will see more buttons - one of them is 'edit' button.
at the bottom of the moot you will see 'add category' button - click on it. Now you can add categories by searching for them or by browsing in the categories browser - and clicking "+" button
you can remove/dissasociate categories from a moot by clicking "x" by the category
if you couldn't find a category - you can add it via 'create new category' button