well, keep the utorrent (or any other bit torrent application opened) from where u finished d/loading the file, right click on the particular file adn there are options to seed..
remember, u also contribute to seeding the file as u d/load it and once u hv completely d/loaded the file, u cann still seed it (for the good sake of the torrent community) to help other ppl d/load the file and this act is known as seeding adn u hv bcome a seeder (once u hv the complete 100% of the file) and u jus seed that is upload it via a torrent application.
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Cheers n e-peace...