On Top click Sensors -> by Type then choose your Sensortype. Check the ones where you want to change your channels and click on settings. In the new window you can choose Channel Settings. Now you can select different Channels and Settings.
Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.