Hello ... I bought a computer in April 2010 and also the microsoft office home student 2007 program. I was told that there will be an upgrade code to 2010 if I registered with my receipt.
All is well with the upgrade. 3 years on and I'm installing the 2007 program onto another computer and it doesn't or I don't remember how to upgrade to 2010.
Many of the old links have been plastered over with microsoft's new sales web site.
Luckily after much digging there was one link left. Here it is.
I think this only works if you still have the old upgrade code.
You'll notice once you get to the download bit that it will tell you how many time you've downloaded the upgrade and when. It is called the... X17-75058

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https://support.microsoft.com/en-gb/help/12435/windows-10-upgrade-faq