Comprend pas le principe des variables...

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

int main(){
    const char *s1 = "Pouet";
    const unsigned n = strlen(s1);
    char *s2 = (char *)malloc(sizeof(char)*n);
    unsigned i;

    for(i=0;i<n;++i){
        if      ('a' <= s1[i] < 'z') s2[i] = s1[i] + 1;
        else if (s1[i] == 'z')       s2[i] = 'a';
        else if ('A' <= s1[i] < 'Z') s2[i] = s1[i] + 1;
        else if (s1[i] == 'Z')       s2[i] = 'A';
    }
    s2[n] = '\0';

    printf("%s\n",s2);
    free(s2);
//  getchar(); // décommente si tu es sous windows
    return 0;
}

Qpvfu

#include <iostream>
#include <string>
//#include <cstdio> // décommente si tu es sous windows

int main(){
    const std::string s1 = "Pouet";
    std::string s2 = s1;

    for(unsigned i=0;i<s1.length();++i){
        if      ('a' <= s1[i] < 'z') s2[i] = s1[i] + 1;
        else if (s1[i] == 'z')       s2[i] = 'a';
        else if ('A' <= s1[i] < 'Z') s2[i] = s1[i] + 1;
        else if (s1[i] == 'Z')       s2[i] = 'A';
    }

    std::cout << s2 << std::endl;
//  getchar(); // décommente si tu es sous windows
    return 0;
}