toolman59 said:
Your esteemed fellow member was very short on facts and very liberal with uncalled for insulting remarks.
OK, as I have done this (and struggled through it) this is what I needed to do (ymmv):

1. You need to have a license for you VM. Your VM is seen as a different computer. If you have not bought a separate license then you may as well stop now.

2. If you have not already upgrade to VirtualBox to version 5 on the host (I have upgraded to Windows 10 on both OSX and Windows 8.1 hosts). I'd recommend it - it works better.

3. In the Guest uninstall the VirtualBox Windows 8 display adapter (otherwise upgrade will fail at 60-70%). This is really boring as it takes about an hour before you get there.

4. upgrade the guest as normal upgrade.

I'd recommend that you download the ISO to use for the upgrade from Tech Bench as the ISO you get via the media tool has a compressed .wim that is less useful - it works OK for upgrade but doesn't work for dism or other MS tools. The Tech Bench version is only about 1GB larger (for x64 en-US which I use).

Now if you want to argue about whether MS should give you a free license for a VirtualBox VM take it up with them (MS - Oracle would not care) but the bottom line is they don't. Any VM needs a license and you need to either buy one or do something else which I'm really not interested in discussing.

As @spapakons says there are free time limited trial versions (90 days normally) you can use if you don't want to buy a full license. If you prefer to do that you can download one from here Virtual Machine (VM), Windows Virtual PC BrowserStack : Microsoft Edge Dev