replace.c

#include <stdio.h>

int main( )
{
    char string[]={"this is a string"};

    char searchchar = 'i';
    char replacechar = 'X';
    char *ptr;    // current position in string

    ptr = string;
    // Loop until end of string
    while (*ptr != '\0') {
        if(*ptr == searchchar)
             *ptr = replacechar;
        ptr++;
    }

    printf (" new string contents are: %s\n", string);
    return 0;
}