Hi Grimtin10,
In order to interact with text files, you should use loadStrings() and saveStrings(). Check out the reference here:
https://processing.org/reference/loadStrings_.html
loadStrings() saves a text file that is in your sketch folder to an array of Strings. If you know where the character you want to change is, go in to that array, change the character, and then save the String array using saveStrings(). You can save it under the same name if you want to overwrite the original file.
Best of luck.
- Trilobyte