I think right now the download speed is quite slow from Microsoft, it's only around half the speed I'm getting compared to when I downloaded during midnight 2 days ago.
I got careless though, just created a bootable USB drive without making a backup of the ISO file, and my Win10 failed to load up after I updated my graphic card driver. I then realized I created the bootable drive for BIOS format instead of UEFI which my laptop is using.
Now I have to re-download the ISO file again.

I found a useful guide, which said you can use non-Windows OS, or use your mobile phone to access the link below:
https://www.microsoft.com/en-us/software-download/windows10

And instead of redirecting you to the link to download media creation tool, you can actually select Windows 10 version and 32, 64bit version ISO file to download directly from Microsoft server. In that case, you can use download manager to download, instead of through the Media Creation Tool.

Read the link below for detailed info:
http://www.askvg.com/tip-get-direct-downlo...from-microsoft/

Side note : How do you do that?

I've recently installed windows 8,1 on my new system.

Already ran windows 8,1 update to the latest version.

What should I do to upgrade to windows 10?

Download the ISO myself and upgrade?