how to convert lower case to capital in c code example

Example 1: C string convert All Uppercse To Lowercase

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

int main() {
   char s[100];
   int i;

   printf("\nEnter a string : ");
   gets(s);

   for (i = 0; s[i]!='\0'; i++) {
      if(s[i] >= 'A' && s[i] <= 'Z') {
         s[i] = s[i] + 32;
      }
   }

   printf("\nString in Lower Case = %s", s);
   return 0;
}

Example 2: convert string to lowercase in c

// include all the libraries  used in the program.
#include <stdio.h>
#include <cs50.h>
#include <ctype.h>
#include <string.h>
#include <math.h>

// Base Function
int main(int argc, string argv[])
{
    if (argc != 2)
    {
        printf("Usage: ./substitution key\n");
    }
    else if (strlen(argv[1]) != 26)
    {
        printf("Key must contain 26 characters.\n");
    }
    
    string key = argv[1].toupper();
    string key = argv[1].tolower();
}