Best way is to create a ISO using the MCT then just mount it under Windows 8.1 and run setup.exe from the mounted ISO. Save the ISO in case you need it again or use Rufus to create a bootable UEFI/GPT flash drive you can use top reinstall or do a clean install at some point.
The installation media includes both Home and Pro versions of Windows 10.
Select either USB flash drive or ISO file (see screenshot below).
If you previously selected the ISO file option and once the ISO file is created, select the Open DVD burner option (see screenshot below).
Both the 64-bit installation media (USB flash drive and DVD) can be booted in both UEFI and Legacy BIOS boot mode. Both the 64-bit installation media are also compatible with Secure Boot.
Here is info about UEFI Boot Mode (installing using the GPT partition style) and Legacy BIOS Boot Mode (installing using the MBR partition style). -> link
I used the upgrade this PC now option. Pretty straight forward with less to go wrong, IMHO. If you have more than one PC to upgrade creating bootable media may be a better option though. You can still run the MCT a second time, or third time, and save an ISO and make install media.
In contrast, I had an HP laptop "trashed" by the Win10 upgrade -- and I tried all the methods available. After I spent an hour on hold waiting for an HP support rep, I was told they are being swamped with calls about similar problems -- and are advising their customers NOT to upgrade at this time.