>I have a file named myfile.txt
>
> 1- How can go to a line of this file ?
>
> 2- How can I replace a line of this file?

It depends what kind of file is it. IF it's fixed lenght file you can calculate offset based on record lenght and line # you want to move to. Than you can use FWRITE() to change the record. Otherwise you can use FGET() to read file line by line and write it out into new file replacing record you want in the process.