I don't know if there is an automated way of doing this, but you can go to C:\ProgramData\NCH Software\Inventoria\Categories (for a Windows install [i'm sure a Mac install will place this in on of the Libraries directories]) and change the files names so that they are in the numerical order you desire.
These files are named 1, 2, 3, etc. If you open the files with a text editor, you will see they are simple text files containing categories and subcategories.