There is no one-size-fits-all answer to this question, as the steps you need to take to make Office 365 your default will vary depending on your operating system and the specific applications you are using. However, in general, you can make Office 365 your default by opening the application and going to the File > Options > General tab. From here, you can select Office 365 from the drop-down menu next to the "Default file location" option.