If you are trying to change product keys and it does not seem to be changing and you are not able to activate properly due to this, 

1. Sign out of the Office applications and close all of them.

2. Windows key + R -> type cmd to open the command prompt window as administrator.

3. Input the following command: 

cd C:\Program Files (x86)\Microsoft Office\Office16. 

If you can't find the path, type cd C:\Program Files\Microsoft Office\Office16 (If you are using Office2013, change Office16 to Office15)

cscript ospp.vbs /dstatus (copy the last five digits of the product key)

cscript ospp.vbs /unpkey: the last five digits of the product key

4. Restart computer and reactivate Office.