C :

#include<stdio.h>
#include<string.h>
 
void main(){
    char a[100];
    char c1,c2;
    int i;
    scanf("%s %c %c",&a,&c1,&c2);
     
    for(i=0;i<strlen(a);i++){
        if(a[i]==c1){
            a[i]=c2;
        }
    }
    printf("%s",a);
}

C++ :

#include <bits/stdc++.h>
using namespace std;

int main(){
	string s;
	string w,t;
	int p;
	cin>>s>>w>>t;
	
	p = s.find(w);
	while(p != -1){
		s.replace(p,1,t);
		p = s.find(w); 
	}
	
	cout<<s<<endl;
}

Java :

import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		String str = sc.nextLine();
		String[] arr = str.split(" ");
		String str1 = arr[0].replace(arr[1].charAt(0), arr[2].charAt(0));
		System.out.println(str1);
	}

}