The default way to install updates for your Office packages is through the Windows Update. It's the safest way to do it, however, if you have issues, download the 32/64bit package directly from the Microsoft website. Before installation, make sure the Office package or any other software related to Microsoft is turned off, then apply the update. Reboot the computer and then attempt to install the update again to verify its correct installation. If everything went well, the installer should say the update is already applied.