[racket] changing source code with application still running

Here's a quote from Paul Graham --- quoting from
http://bc.tech.coop/blog/040223.html.

"When one of the customer support people came to me with a report of a
bug in the editor, I would load the code into the Lisp interpreter and
log into the users' account. If I was able to reproduce the bug I'd
get an actual break loop, telling me exactly what was going wrong.
Often I could fix the code and release a fix right away. And when I
say right away, I mean while the user was still on the phone. "

I have been trying to understand *how* this is done. I'd hope that
this is also possible in Racket. But let me ask about the following
specific situation.

Say I have some application running. I told the shell

     ./run_my_application &

Now, while using it, I notice a bug somewhere. How do I change the
code and see its new effect without shutting down and rerunning the
application?

Thank you.