If you mean the machine-id, it is meant to provide a unique id for the machine.
You can edit the machine-id if you like, it is located in %APP_DATA%\Duplicati\machineid.txt or ~/.Duplicati/machineid.txt for most machines.
You can also set it in advanced options, but editing the text file means it will also be used for any commandline initiated backups automatically.
If you set it to something else, you should make sure that you do not have two machines that have the same id, or they will display as a single machine in the portal.