If I upgrade to windows 10 will all my files I have saved on windows 8.1 transfer? or do i have to back it up first? can i torrent on 10 ?

When upgrading to Windows 10 from Windows 7, 8 or 8.1:
- Your DATA files will remain in the same place where you left them before the upgrade
- Some of your programs will keep on working in Windows 10, without needing to reinstall them.
- Everything that couldn't be upgraded to the new system, will be found in C:\windows.old where you have the usual directory structure with Program Files, Program Files (x86), Users, Windows and so on.

You should keep the folder C:\Windows.old for a while until you are really sure you do not want to go back to Windows 8.1. You are safe to delete it, but you will lose the option to revert back to your previous system in case you have an issue with Windows 10.

It is ALWAYS best to make a backup of you files. I've seen it several times that a hard disk crashed during the update (or some other windows update) which renders your data useless. Also very few people report issues during the upgrade which eventually require a reinstallation. This also contains a risk of losing your data.
And yes you can torrent on Windows 10. I highly recommend the opensource qbittorrent instead of utorrent (contains spyware these days).

All your files should be saved but it is ways wise to do a backup first. Anything you can do with Windows 8.1 you can do with Windows 10.