How to overwrite strings for a file at a specific position

Saying that I have such a file as below:

abc
def
2
3
1
fgh

I need to:
1) find the first letter f and append a string mark;
2) find the second letter f and replace it with a string mark2;
3) make the numbers in order.

Meaning that the file above should be like this:

abc
defmark
1
2
3
mark2gh

I've learnt that we have functions like seekg and seekp but I've no idea about how to use them. Also, I think I can read the numbers into an array, make them in order and overwrite them but I don't know how.

Or maybe I shouldn't use C++ to do this? But I've tried Linux script and it seems to be very complex too.