unix command to reverese lower and upper case



im trying to find a command to
change all upper case to lower case and
all lower case to upper case.

for example:
file is 'red sox'
need it changed to 'red sox'

tried tr 'a-za-z' 'a-za-z' didn't work

what an i missing??

i tried


code:
tr "a-za-z" "a-za-z"