Toupper() function is used to convert the value to uppercase when it used with characters.

Code:

#include <stdio.h>
#include <ctype.h>
int main()
{
     char c;

     c = 'a';
     printf("%c -> %c", c, toupper(c));
     
     c = 'A';
     printf("\n%c -> %c", c, toupper(c));

     c = '9';
     printf("\n%c -> %c", c, toupper(c));
     return 0;
}

Result: