I actually recorded the video about how I did upgrade Windows 8.1 to Windows 10 on July 2015, but only got time to edit and published it recently.
There are several things you should check first, before you run the processes.
These are things that I would check, before running the process to upgrade Windows 8.1 to Windows 10.
1. Check if your computer has the requirements to upgrade.
2. Backup your important files from your computer, just in case, things turns into unexpected result.
3. You should have your license key of your current OS you have in your PC.
4. Get all the updates of Windows 8.1 before you start to upgrade Windows 8.1 to Windows 10.