That is relatively easy …
The basic FAQ way:
If you have copied the data file into the default download folder before you reload the torrent into your torrent client programme, the programme will find it there. You may need to run a recheck.
The µTorrent/BitTorrent pointing to any other location way:
1/ Load the torrent and don't start it!
2/ Click with the right mouse button on the torrent in µTorrent's list and select "Advanced" > "Set Download Location…" > navigate to the file location and confirm (Save)
3/ Right mouse button click on the torrent in µ/BitTorrent's list and select "Force re-check". The torrent should check to 100% and µTorrent displaying "Finished" instead of "Stopped" in the status column.
4/ Start the torrent, it shall go to "Seeding" or at least to "Queued seeding"
or
1/ Load the torrent, µTorrent/BitTorrent will show a small window with the target download location. Click there the ". . ." button.
2/ Then navigate to the file location and confirm (Save). Don't start the Torrent.
3/ Right mouse button click on the torrent in µ/BitTorrent's list and select "Force re-check". The torrent should check to 100% and µTorrent displaying "Finished" instead of "Stopped" in the status column.
4/ Start the torrent, it shall go to "Seeding" or at least to "Queued seeding"
Both ways are equal.