The 851N can certainly play files at 24-bit/192kHz.
First, make sure that the 851N is set to USB Audio class 2.0 mode. This is done via the Audio settings menu on the front display.
Setup for PC:
Make sure to install the appropriate Cambridge Audio Windows 2.0 USB driver on your computer. Information on which driver to install can be found below. Once you have finished the installation, go to the Sound menu in Control Panel and set the Cambridge Audio USB 2.0 product as the default output device.
Setup for Mac:
If you are using a Mac you will not need to install any additional drivers. Simply navigate to System Preferences > Sound and select the 851N as the output.
Information on which driver to install and the advanced settings you can amend can be found in the document below.
The required driver can then be downloaded here.