To open new URL in the same popup window, from your code, I edited to below:
var win = window.open("http://localhost:8000/player/index.html", 'newwin', 'height=200px,width=200px');
after run this line, new popup window will be showed with height=200px, width=200px. To open new URL in the same popup windown (win), use this line
win.location = "http://www.google.com"/
You can replace google URL by any URL you like.
Hope it helpful! Good luck. Johnny