How to change product key in Office 2016.

There is no option to change product key in "Add/Remove programs" trough control panel.
Instead of using Control Panel, run command prompt with administrative privileges and run following commands:

to check key status:

For Office 2016 32bit on 32bit version of Windows
cscript "C:\Program Files\Microsoft Office\Office16\OSPP.VBS" /dstatus

For Office 2016 32bit on 64bit version of Windows
cscript "C:\Program Files (x86)\Microsoft Office\Office16\OSPP.VBS" /dstatus 

For Office 2016 64bit on 64bit version of Windows
cscript "C:\Program Files\Microsoft Office\Office16\OSPP.VBS" /dstatus

you'll get installed license details and last 5 character of Product Key.

you can change product key by entering command /inpkey:value for example:

cscript "C:\Program Files\Microsoft Office\Office16\OSPP.VBS" /inpkey:XXXXX-XXXXX-XXXXX-XXXXX-XXXXX