e.g.

https://forum.processing.org/topic/check-and-replace-certain-char#25080000001371625

also have a look at

http://processing.org/reference/String.html

there is stuff like indexOf and you can thus get the position of the comma and then with twice using substring go like

1. String newString = subStringFirstPart + "." + subStringSecondPart;

Greetings, Chrisir