How rewrite first line of a file ?

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C++

How can i rewrite the first line of a line with out making a copy of file and rewriting it. I am using sequential writing for example my file contains data

hey<br />
how are you everyone at codeproject??

and i want to rewrite it as

hello!!<br />
how are you everyone at codeproject??

i tried this code

ofstream fout2("res.dat",ios::app);
	fout2.seekp(0, ios::beg);
	fout2<<str;
	fout2.close();

but it write str to the end of the file :(

Any suggestions ?