There's great documentation on the arch wiki about swapping keys using xmodmap:
xmodmap - https://wiki.archlinux.org/index.php/Xmodmap
To swap keys with xmodmap you need to know the key's keycode or scancode.
You can find out key codes using the program xev in terminal.
finding keycodes with xev- https://wiki.archlinux.org/index.php/Extra_Keyboard_Keys
When you know the key's code you can make temporary changes using xmodmap in terminal:
xmodmap -e "keycode 46 = l L"
(Actually this example from the arch wiki makes me think you can change some keys without knowing the keycode.)
xmodmap -e "keysym a = e E"
These changes only last till you log out.
When you're happy you have determined the changes you want to make, you can save them to a file in your home directory:
The Arch wiki suggest you need to perform additions steps to have this file load on login, but my experience with 13.04 is that the file loads automatically.