Want to do something other than play games on your Porta Pi Arcade? That is dumb, but if you must, it's very simple to install Rasbian/ Pixel in top of you RetroPie OS. The file manager is a great way to weed through your rom collection btw...
Note: RetroPie v4.x has been giving me woes, so this tutorial is done with RetroPie v3.8x. Follow the pics to install the newest Linux disto onto your Raspberry Pi.
Setup: Raspberry Pi 3, Class 10 SDcard (8GB) wired ethernet connection.
Estimated tiem of burden: Took about 30min? I wasnt really tracking time, just walked away from it then checked back...
RetroPie Setup Script has all sorts of goodies.
This will take some time to grab files and install. Go make a sandwich.
When it's all done, exit to terminal.
Type startx (above the bald spot) and press enter. Done.