Someone

int charreplace(char * string, int find_character, int replace_character) {

	int i, length, found;
	
	length = strlen(string);
	found = 0;

	for (i = 0; i < length; i++) {
		if (string[i] == find_character) {
			string[i] = replace_character;
			found++;
		}
	}

	return found;
}

int main(int argc, char *argv[]) {
	
	char * query_string = "cmd=and%20then%20the%22cat%22";
	int replacecount;

	printf("original string : %s\n", query_string);

	replacecount = charcount(query_string, '%');
	printf("%d occurrences to replace\n", replacecount);

	replacecount = charreplace(query_string, '%', 'Z');
	printf("%d occurrences replaced\n", replacecount);

	printf("replaced string : %s\n", query_string);
	//replace_hex_with_ascii(query_string);

	return 0;
}