Hi @Steven Willett. You can save the current configuration first for you to be able to edit it.
Click File (on top)
Click Save configuration as...
Save the configuration file on a folder which you can access easily (desktop would be fine)
After saving, open the configuration file with Notepad
Edit the configuration according to the setup you want
Save the configuration file
Open Synergy interface
Click Use existing configure
Choose the file you have saved
Click Apply