Changing key bindings of printable chars

[Paul Batt]

How can I change the key binding of a single printable character key to
another? I need to have "z" and "Z" print "y" and "Y" and vice versa and I
need to have RET do a double-return e.g. two line feeds instead of only one.
(I found many peaces of instruction how to change key bindings of double
key-strokes like C-x but can't just figure out how to do it for a single
printable character key). Thank you.