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

void main()
{
FILE *fp,*fout;
int i=0,len_string;
char SearchText[]="Hello"; /* You can replace this text */
char ReplaceText[]="Help"; /*You can also replace this text. */
char temp[30];
fp=fopen("log.txt","a+");
fout=fopen("temp.txt","a+");
rewind(fp); /* for going to start of file. */
if(fp==NULL || fout==NULL)
{
printf("File couldn't be opened ");
exit(0);
}
len_string=strlen(SearchText);
while(!feof(fp))
{
for(i=0;i<len_string;i++) temp[i]=fgetc(fp);
temp[i]='\0';
if(stricmp(SearchText,temp)==0) /* the stricmp() is used for comparing both string. */
{
	
fprintf(fp,"%s ",ReplaceText);
fprintf(fout,"%s",ReplaceText);
fflush(fp);
fclose(fp);
fclose(fout);
exit(1);
}
fseek(fp,-(len_string-1),1);
}
fclose(fp);
fclose(fout);
}

Original: 
"Welcome to C. I love programming."

 "Welcome to C. I lovelike ogramming."

An append would be:
"Welcome to C. I love programming.like"

A replacement would be:
"Welcome to C. I like programming."

file = open("myfile.txt", "r+")
data = file.read().replace("texttoreplace", "newtext")
file.seek(0)
file.write(data)
file.close()

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

int main(void)
{
   char *str1 = "Hello World", *str2 = "or", *ptr;

   ptr = strstr(str1, str2);
   printf("The substring is: %d\n", ptr-str1);
   return 0;
}

//searches a file for a target string. If it's found, it appends a string, onto the file.

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

int main() {
  FILE *fp;
  int gotit;
  char *ptr;
  char buff[BUFSIZ];
  char target[]= "love"; 
  char append[]="like"; 
  
  fp=fopen("log.txt","at+");

  if(fp==NULL)  {          
    printf("File couldn't be opened ");
    return 1;
  }
  gotit = 0;
  while(fgets(buff, BUFSIZ, fp)) {
    ptr = strstr(buff, target); 
    if(ptr) {
      printf("\n The substring starts at the %d char", ptr-buff);
      printf("\n Target string was found, appending replacement text");
      rewind(fp);
      fprintf(fp, "%s", append);
      gotit = 1;
      break;
    }
  }
  fflush(fp);
  fclose(fp);

  if(!gotit) 
    printf("\n The target string was not found");

  printf("\n\n\t\t\t     press enter when ready");
  gotit = getchar(); 
  return 0;
}