The simplest would probably be to uninstall and reinstall.
Alternatively, you need to find the settings.xml file that stores this information.
If you are using the zip file, then it will be found in the same folder that you unzipped the application to. If you are using the ClickOnce release then you'll probably need to search for it - for me on Windows 7 it was here:
With the application closed, replace the <UseGoogleCalendar> section with:
<Id i:nil="true" />
<Name i:nil="true" />