Hello Holly,
You really should take advantage of upgrading to Windows 10, as you are a windows 8.1 user you are entitled to the free upgrade. The Windows 10 Operating system is not part of Office 365, Office 365 is the Microsoft Software Suite of Word, Excel, Outlook etc.
Here is a comprehensive guide to creating Windows 10 upgrade media,before performing the upgrade be sure to backup all important information to an external hard drive.
Best of luck, hope that helps!