First you need to find out what the keycode is for the key you want to map. Bring up a terminal and type in the following, followed by the Enter key:
Then press on the Right Alt key. In the terminal you will see something like this:
KeyRelease event, serial 31, synthetic NO, window 0x3800001,
root 0x5e, subw 0x0, time 9174459, (-616,-31), root:(482,587),
state 0x8, keycode 113 (keysym 0xffeb, Alt_R), same_screen YES,
The part you need to know is the keycode which here is 113.
The other thing you need to know is Super_L is the name of the left windows key.
Close xev and in the terminal go to your home directory and make a file called .Xmodmap. I doubt that you already have one. Use whatever text editor you know, but don't use a word processor. I show using nano here.:
In the file type. Substitute the keycode that you found in xev for the 113:
keycode 113 = Super_L
After this, log off and back on to incorporate the new addition to your keymapping. After this when you run xev and press the key, you should see Super_L instead of Alt_R.