If you already have Office 365 Installed, you can add an additional account using the guide HERE
If you have an older version of office, you can uninstall that version and install the newer version, if you have other Outlook Email accounts installed, ensure that you have all the server details for those accounts and your signin details for those accounts.
If you do not have office installed, complete the following Steps
Once inside the office website, find Install Office
Click on the little Down arrow and select Office 365 Apps