Which Adagio program are you referring to? OrderEntry? Receivables? Another? The application in use would determine the correct way to set this up.

In OrderEntry, tax statuses are defined on the item level, not the category level. So each item would have to be modified if you want to change these. Take advantage of the Export/Import Items functions so that you could export all the items to an Excel spreadsheet that you want to change, make the changes in Excel, then re-import back those items.