Here is the instructions for change the Office 2019 / 2016 Pro Plus key:
1- Open any Office app, select the File menu and then select Account near the bottom of the menu.
2- Under Product Information, select the Change License button.
3- After changed your license key you will find your new Installation ID so you can submit your Installation ID from our Get Confirmation ID page:
Remember, sometimes Microsoft servers are stuck so if you can’t get your Confirmation ID within 30 second please refresh the page and submit again.
4- After you enter your Confirmation ID your activation will be completed successfully.