Microsoft never make it simple. I purchased a windows 8 upgrade for 25 quid to upgrade my existing windows 7 install. However my SSD with win7 on it died so I installed windows 8 onto a new SSD. But then windows wouldn’t activate as it was a ‘clean install’ rather than an ‘upgrade install’.  Seems like this is a common problem but easily sorted

• Open regedit by pressing Windows-q, entering regedit and selecting the result from the list of hits.
• Navigate to HKEY_LOCAL_MACHINE/Software/Microsoft/Windows/CurrentVersion/Setup/OOBE/
• Change MediaBootInstall from 1 to 0
• Go back to the start screen and enter cmd there.
• Right-click Command Prompt and select to run it as administrator.
• Type slmgr /rearm on the command line and hit enter.
• Reboot Windows now.
• Run the activation utility afterwards, enter your product key to activate Windows.