C语言订单系统客户隐私安全的个人信息更新功能开发求助

我正在开发一款简易订单系统，需要实现客户个人信息更新功能。目前我的代码会打印整个客户信息文本文件（custdeet.txt），让用户选择要更新的行，这会导致客户能查看其他用户的隐私信息，我无法解决这一问题。

注：我曾用类似方式实现菜单菜品更新功能且运行正常，但作为编程新手，我找不到更好的改进方案。以下是当前陷入瓶颈的函数代码：

void ChgDeet()
{
    FILE *cd1;
    FILE *cd2;
    int line = 0;
    int temp = 1;
    int flag = 0;
    char fname[50];
    char lname[50];
    char email[250];
    int number;
    char d;
    printf("\nNAME\t\tEMAIL\t\t\tPHONE NUMBER\n");
    cd1 = fopen("custdeet.txt", "r"); // open file in read mode
    if (cd1 == NULL){
        printf("\nUnable to open file.\n");
        quit();
    }
    while (!feof(cd1)){
        d = getc(cd1);
        printf("%c",d);

    }
    //take cd1 to start point
    rewind(cd1);

    printf("\n\nEnter line number to be updated: ");
    scanf("%d", &line);
    fflush(stdout);
    //open duplicated file in  write mode
    cd2 = fopen("copy.txt", "w");

    while (!feof(cd1)){
            d = getc(cd1);

            if (d == '\n')
                temp++;
            //until the line to be deleted comes, copy content from one file to the other
            if (temp != line){
                putc(d, cd2);
            }
            if (flag == 0 && temp == line){
                printf("\nUpdated First Name: ");
                scanf("%s",&fname);
                printf("Updated Last Name: ");
                scanf("%s",&lname);
                printf("Updated Email: ");
                scanf("%s",&email);
                printf ("Updated Phone Number (+60): ");
                scanf("%d",&number);
                fprintf(cd2, "\n%s\t%s\t%s\t+60%d", fname,lname,email,number);

                flag = 1;
            }
        }
        fclose(cd1);
        fclose(cd2);
        remove("custdeet.txt");
        rename("copy.txt","custdeet.txt");
        printf("\nUpdate Menu: \n");

        cd1 = fopen("custdeet.txt", "r");
        if (cd1 == NULL){
            printf("\n Unable to open file.\n");
            quit();
        }

        while (!feof(cd1)) {
            d = getc(cd1);
            printf("%c", d);
        }
        fclose(cd1);

    printf("\nYour personal information updated!");
    printf("\n...redirecting you back to homepage...");
    CustMenu();
}

内容的提问来源于stack exchange，提问作者Kemmy