Simple code

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

extern void subs (char *string, char this_c, char that_c) ;

int main (int argc, char *argv[])
{
        char this_c = 'e';
        char that_c = 'X';
        char orgstr[] = "sir sid easily teases sea sick seals";

        for (i:orgstr)
        {
        if(i == this_c)
                {
                i = that_c;
                }
        }
        printf("Changed string: %s\n", orgstr);
        exit(0);
}