How do i Replace every n-th character x from an String

My Question would be how can replace every 3rd ';' from a String a put a ',' at this position ?

for eg.:

String s = "RED;34;34;BLUE;44;44;GREEN;8;8;BLUE;53;53"

so that the String looks like:

RED;34;34,BLUE;44;44,GREEN;8;8,BLUE;53;53

I tried to solve it like this but i can't take a charAt(i) and replace it with an other char.

int counter =0;
for (int i=0;i<s.length();i++){
    if(s.charAt(i) == ';'){

    counter++;
    }
    if(counter ==3){
    s.charAt(i)=',';
    counter =0;
    }


}