Settings window -> Game tab -> "Register" button -> "Remove License" button. If you are connected to the internet when you do this, it will disassociate the license with that PC, allowing you to install the license on another PC. The licensing server does not collect information about your PC (such as what it is named, or what the network interface address is), so there's no way to for you to identify which PCs have your license installed based on the licensing server data.
In cases where you've run into the limit (for whatever reason), email support@thetournamentdirector.net. I'm happy to help.
No one in the forums can help you with this, which is why there is a rule against posting about licenses.
Please email support@thetournamentdirector.net.