If the computer was activated there is no longer a need for a license key as long as you are installing the same version of Windows 10.
If asked for a key just go to the bottom of the page and click on Skip, Do this later, or I do not have one.
To get updated to the latest Download the Media Creation Tool and upgrade.
Media Creation Tool, US;
https://www.microsoft.com/en-us/software-download/windows10
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Media Creation Tool GB;