Hai guys i'm doing a project in C++.i want to know how to update the contents of a file in c++.?

If you mean updating the data that is already in the file (for instance, changing a word in the file to another word), then you'll need to open the file for reading, read the whole file into a string (or several), parse the data, edit the data, and then write it back to the file. Essentially, this is what games and applications do for INI files and such.

If you just mean updating as in adding new data to the end, then just using the append flag when opening your fstream.

I am not positive, but if you are talking about a flat - or "text" file - I don't know of any easy way to actually 'update' the contents of it. Typically, in situations such as these, the file is simply overwritten with the new, modified file.
Unless there is some specific reason for which you need to actually modify the contents (e.g. for a class assignment or something), I would suggest simply overwriting the old file with the new, updated one.

Good luck : )