Windows 7 change product key and activate from command line

Open CMD as Administrator (elevated).
cd /d %systemroot%\system32
cscript slmgr.vbs /inpkey:XXXXX-YYYYY...
cscript slmgr.vbs /ato


Windows 10/2016/2019:

cd /d %systemroot%\system32
cscript slmgr.vbs /ipk XXXXX-YYYYY...

Good luck.