There are a couple of ways to accomplish this:
Method 1
On UNIX/Linux clients, the guid is stored in the /opt/altiris/notification/nsagent/etc/host.GUID file.
Method 2
Uninstall and reinstall the agent on the client computer.
If either step does not work, select and delete the corresponding entry in the resourcekey table for the erroneous GUID and repeat the process.