Do you mean to use a regular USB-to-serial, or the Siemens PC Adapter USB ?
The 1st one isnt possible with the TP177B.
The 2nd one should be possible to select if you open the PG/PC Interface, and select S7ONLINE = PC Adapter, and then in the settings for the adapter specify USB.
edit: Do you have both STEP7 and WinCC Flexible, or only WinCC Flexible ?
If you have STEP7 then the latest drivers incl. the one for the PC Adapter USB will already be installed. Not sure when you only have WinCC Flexible. Maybe, maybe not.
edit again: It just dawned on me that maybe you want to attach the TP177B as a USB device on your PC. I am not sure if that is even possible.