First of all, make sure you have a stable internet connection, as activation is directly dependent on it. If you have a product key, open any Office application, such as Word, and from the top menu click on "File", then "Account" and "Activate Product". Enter your product key and follow the on-screen instructions. However, if this doesn't work, your key may have already been used or locked. In this case, there are a few workarounds. One of them is to use specialized programs such as windows 10 activator tool. This application helps you activate not only Windows, but also Office products. You need to download this program from the developer's official website (careful not to fall for fake sites), run it and follow the instructions. The process usually takes only a few minutes. Also note that using such tools may not be completely legal, so it's best to try to resolve the issue through official Microsoft support channels first.