Update file using fscanf and fprintf

I have tab seperated record like this in .txt file

1000    Aashir  0213-4211685    123456  0
1001    Muhammad Ali    0334-3387918    abcd    0
1002    Azam    0323-2143133    hassan123   0
1003    Taha Hashmi 0214-6589421    786 0
1004    Khan    03452145698 helloworld  0

now i want to update or modify these values using fscanf and fprintf

This is what I've tried so far but now i got stuck on how to write on a specific position

int update_record(Account* user)
{
    FILE *record;
record=fopen("database.txt","a+");
char buffer[SIZE];Account req;
while(fgets(buffer,SIZE,record))
{
    sscanf(buffer,"%d\t%    [^\t]\t%s\t%s\t%ld\n",&req.acc_num,req.name,req.mobileno,req.pass,&req.acc_bal);

    if(req.acc_num==user->acc_num)
        break;
     .
     .
     .
}

I am beginner please help, i have to use fscanf and fprintf only, #assignmentcriteria :(