Joining a torrent with qBittorrent
One of the most used methods to update torrents is saving the new contents on top of the old contents, but doing that with qBittorrentis not so intuitive. Below is an explanation of how to do that.
Folder
The window that appears when you add a torrent is below with an example:
At first sight, it appears that the files would be saved to folder F:\PD but that is not right.
The files are saved on the path resulting of the combination of the "Save at" section and of the top-level folder that is on the right panel.
In the example, the files would be actually saved to folder "F:\PD\MAME - Update ROMs (v0.227 to v0.228)".
The top-level folder can be renamed by right clicking it. Another option is ignoring it and saving the files directly to folder F:\PD by disabling "Keep top-level folder" on the left.
Hash check
If files might be missing or different from the contents of the torrent, keep "Skip hash check" disabled. It will take longer to check what you miss, but you will download only what is necessary to have a perfectly matching set of files.
If you are absolutely sure that you have 100% of the torrent contents (besides qBittorrent itself, TorrentCheck can help you verify that), you can enable "Skip hash check". When clicking on OK, qBittorrent will only check if all files are there, but it will not check the contents of the files. This is almost instantaneous and will make you start seeding earlier.