Replacing all characters in a .txt file with another character [on hold]

I wrote the code to read in a .txt file, and then to print it into a new output file. I am trying to replace every character in the .txt file with a randomly generated character, but have no clue where to start. So for example, every "a" found in the text will be replaced with a "f" and every "c" will be replaced with an "h" and so on and so forth.

I am writing this in C++