The order of categories within your app can be ordered via the 'App Categories' section of your dashboard.
Hovering over your categories in this list will display an option to edit the category.
From the edit menu you can see a list of options, including the 'Order' option.
Changing the number in the Order field will adjust the position of this category within the app. All categories will default to the order value of 0.
The order value works from lowest to highest, with the lowest number having the highest priority. Here's an example to help you understand:
Here you can see that the Alerts category displays at the top of the list, as its order value is -10. Making it the lowest value in the list.
Next is Contact with the value of 0, and then Canteen with the value of 5.
If we wanted the Portal category to display at the top of this list, we would just need to value it something lower than -10, such as -20.