It seems that I can't see the change categories field is there ay particular setting I need to turn on ?
I actually found that there is a setting to either show or hide the categories in a change ticket
Where in the system are you looking when you try to change categories? You should be able to hover over these fields at the top of the ticket and it will show a dropdown icon, letting you change the category and subcategory.
If you mean you are trying to change what options there are for categories and subcategories, that will be under Setup > Service Desk > Categories.
