You can access the configuration file going into Parsec's settings, scrolling to the bottom and clicking "edit the configuration file directly". These are available on all platforms, except Android.
Alternatively, you can find config.txt on your system's files:
- macOS / Linux / Raspberry Pi:
You will need to open the Parsec configuration file directly in Notepad and run as an administrator. On shared install systems, you can find the config file here:
%programdata%\Parsec\config.txton Shared installations
You can see a list of the available configuration files here.