How to seed with a file already downloaded

[zaza64]

Hi !

As a newbie, I have downloaded some files and once finished, I have cancelled the downloading. But now, some of my trackers doesn't allow me to leech ... (message is : access denied, leeching forbidden, you are only allowed to seed). How could I seed some files I have ? I have seen some old tutorial which show how to seed by selecting an existing tracker and the downloaded file and then utorrent "understand" you want to seed not leech. But I have not found anymore this in my version 1.8.3.

Thank you all for your help

Best regards

Sylvain