How to uninstall an update

You cannot uninstall an update or upgrade. You need to start by backing up your files, erasing the drive, then reinstalling macOS. Unless you have a copy of the Yosemite installer you cannot reinstall it. You can only download a copy if you have downloaded it from the App Store in the past. In this case, you should find it listed on the Purchases page of the App Store app on your computer.

Install Lion Through Yosemite from Scratch

Be sure you backup your files.

1. Restart the computer and after the chime press and hold down the Command and R keys until the Apple logo appears. Wait for the Utility Menu to appear.
2. Select Disk Utility from the main menu and click on the Continue button.
3. After Disk Utility loads select the volume (this is the indented entry, usually Macintosh HD) from the side list. Click on the Erase tab in Disk Utility's main window.
4. Set the Format type to Mac OS Extended (Journaled.) Click on the Apply button
5. When the process has completed quit Disk Utility, and return to the Utility Menu.
6. SelectReinstall OS X and click on the Continue button.

This will install the version of OS X you had installed.

Time Machine Basics

Time Machine Basics (2)

Do you have a backup you can restore from? What update did you install? Is Word the only application that is freezing?

It sounds like you downloaded an update to Microsoft Office that now requires you to use Microsoft Office 365. Do you have any kind of backup, such as Time Machine?

Can you close that document and open another one?

I would start making 2 backups on 2 separate drives. They help in this type of situation.