The CB > User Management import feature can be used to mass-update. You just need to change how the import is handled using the dropdown. All you should need to include is their user id in an id column and whatever fields you want to update. Set the dropdown import option to "Overwrite existing entries with records matching ids". It will try to create a new user though if the user id supplied doesn't exist.

Alternatively you can just use database tools for this. phpmyadmin for example supports importing data.