Hi otophile,
For the Pi B2 and B3 no need to edit the config.txt, it should be set up automatically. See:
#44341 and look for the text "
No need to edit config.txt, I2S DAC can be enabled from UI."
For the Pi A, B, B+, Zero or Zero W you can access the config.txt file simply by pluging the SD card into a PC. Part of the SD card is accessable for Windows and Linux and the config.txt file is easy to find. On a Windows machine you can use Notepad to edit the file.
No need to copy files, etc.
Janui
PS It would be handy if you could mention your hardware and image when posting
