How do I replace the pipe that runs from the toilet shut off valve to the wall. It has a tiny leak,

Depends on exactly where the leak is, what kind of pipe is used? Could you post pictures of the pipe in question?

That pipe can be a sweat fitting or threaded, we can't advise you until we see/know how its attached.

See the images below and respond to Bob's qustion.
Which image looks like your shut off valve and where is the leak?

Thnak,
John