Use the Piranesi Licensing Program (isiadmin).
Piranesi will ask for your licence details if you have not entered your licence yet, or if a licence has expired.
If you need to update an existing licence, run isiadmin.exe which is in the Program folder on your Piranesi DVD. Select Piranesi from the "name" dropdown list at the top, then click "Load Licence". Enter your new user limit, validation etc. and click Save.
For network licences, we recommend that each machine reads the Piranesi licence from a shared read-only file. Then you only need to edit this file to change the licence for all users. You can do this when you first enter a licence, or by using isiadmin: on the first machine, enter your licence, uncheck the box "use local machine registry", click Browse, go to the folder and enter a filename, then click OK (or Save) to create the file. On other machines, uncheck the box and Browse for the file. If the file can not be read, Piranesi uses a copy of the licence in the registry.