talexb has shown how this simple substitution works. Note, however, that the output is not "Hacker" as you have it in the OP, but "hacker" (little-h vice big-H). To verify your understanding of what's going on, what simple change to the replacement string of the  tr/// operator would be necessary to make the output "Hacker"? (Of course, this simple change screws up another thing, but that's life... :)