First find the keycode for 'z' and 'y' with xev tool
(in a xterm, type 'xev', then type 'z' and 'y' and
look the terminal output for keycode)
Once you have your keycodes, say
z=25 and y=29, try ;
Code:
xmodmap -e 'keycode 25 = y'
xmodmap -e 'keycode 29 = z'