Step 1.
Open any Office product (e.g. Word/Excel), in this example we will use Word 2016, as the steps are similar in the 2019 and 2016 versions.
Step 2.
Click on "Open other Documents".
Step 3.
In the new menu, click on "Account" or "Konto".
Step 4.
Now click on "Change Product Key" or "Produktschlüssel ändern" (highlighted in red).
Step 5.
In the window, please enter your replacement key and click on "weiter" or "Continue".