Missed out on the free Windows 10 upgrade? You can't have, you can still upgrade to Windows 10 with a Windows 7 or 8 product key. It seems that Windows 10 can still be activated using a Windows 7 or 8 product key, meaning you can install it on any computer that meets the requirements as long as you have a valid Windows 7 or 8 product key. Whilst we're not one hundred percent sure that this will still work for everyone, from what we've tested, it still works. We're not sure why this is but 2 potential reasons could be a) Microsoft is still offering Windows 10 for free to Windows 7 and 8 users who use assistive technologies (more info here) so they decided to continue to allow WIndows 10 to be activated with Windows 7 and 8 product keys and b) it may cause problems with computers running Windows 10 that were activated with a Windows 7 or 8 key.
If you would like to make use of this and try installing Windows 10 with a 7 or 8 product key, you can download the installer here although you should do this at your own risk in case it doesn't work. If it does not work and you would like to restore your previous Operating System, go to Settings > Update & Security > Recovery, you should be able to revert to your previous OS provided the files have not been deleted (note that they are automatically deleted after 30 days).