blabla

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

int main()
{
    FILE*fp;
    char buff[50],user[50],old_user[50],old_pass[50],
    pass[50],new_user[50],new_pass[50];
    printf("Login\nUser:");
    scanf("%[^\n]%*c",&user);
    printf("Pass:");
    scanf("%[^\n]%*c",&pass);
   fp = fopen("C:\\login\\userpas.txt", "r");
   fscanf(fp, "%s", old_user);
   fscanf(fp, "%s", old_pass);
   fclose(fp);

   if(strcmp(user,old_user)==0 && strcmp(pass,old_pass)==0){
       printf("Login Anda Berhasil\n");
   printf("apakah anda ingin mengganti User/pass (ya/tidak)");

   scanf("%[^\n]%*c",&buff);
   if (strcmp(buff,"ya")==0){
        printf("Masukkan User dan Password Baru!\n");

    printf("User Baru:");
   scanf("%[^\n]%*c",&new_user);
    strcat(new_user,"\n");
     printf("Password Baru:");
   scanf("%[^\n]%*c",&new_pass);

   fp = fopen("C:\\login\\userpas.txt", "w+");
   fprintf(fp, new_user);
   fprintf(fp, new_pass);
   fclose(fp);
   printf("User dan Password Berhasil di Buat!\n");
   }
   else{
    printf("Login anda Selesai!\n");
   }
   }
   else{
    printf("Login anda Gagal!");
   }
    return 0;
    }