Altering copy_to_user buffer contents

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hi.
i am developing a small module for the linux 2.6.14 kernel.
so far i have been able to create the character devices that will contain any kind of char data i send them.

for example, if i type

echo HELLO WORLD > /dev/vale0

and then

cat < /dev/vale0

then the output will be

HELLO WORLD

but now i would like to alter the result of the last cat comment.
let's say i want to swap the letter O with the letter X so that the output will be:

cat < /dev/vale0

HELLX WXRLD

i suppose the main task is to modify the copy_to_user buffer, but I have absolutely no idea of doing it.
any suggestion will be gladfully accepted.

thanks guys