How can i use another main line outputs in stereo

The default output channel in streaming mode depend on the driver. If you are using ASIO connection you can select the default output in the multimedia application (like Foobar, etc).
If you are using standard WDM, you can select on your windows control panel / sound / playback, here you can set the default stereo output.

If your LP-16 first channel have different sound compared to the another channels, please contact with the shop where you bought the LP-16 and ask replacement device, if your device under the warranty.