https://support.office.com/en-us/article/Change-your-Office-product-key-d78cf8f7-239e-4649-b726-3a8d2ceb8c81

1. Open elevated Command Prompt
2. Find last five characters of current Key –
   cscript “C:\Program Files\Microsoft Office\Office16\OSPP.VBS” /dstatus
3. Remove current key –
   cscript “C:\Program Files\Microsoft Office\Office16\OSPP.VBS” /unpkey:XXXXX
4. Enter new key –
   cscript “C:\Program Files\Microsoft Office\Office16\OSPP.VBS” /inpkey:XXXXX-XXXXX-XXXXX-XXXXX-XXXXX