How do I downgrade from the latest Catalina update?

What you propose will be difficult and may be impossible to achieve, since Apple has made it difficult to impossible to reinstall that Mac's originally installed operating system.

BDAqua outlined what's in store for you if you decide upon that option. Instead of doing all that, consider any one of the many alternatives to Microsoft's bloatware. Of course you can choose to buy Microsoft 365 or whatever they call it these days, but Microsoft will never stop squeezing you for more 💰.

Consider LibreOffice or similar MS Office type programs. They are not expensive (some may even be free) and will read and write Microsoft formatted documents.

https://www.libreoffice.org/

Personally I use Apple's Pages and Numbers, they're free, and they also read and write their Microsoft equivalents.

‎Pages on the App Store

‎Numbers on the App Store

Life is better without Microsoft.

Whatever OS you want to go back to will require Erasing.Reformatting the Hard Drive...

In Disk Utility>View, select Show all Devices, highlight the top left entry.

Run Disk First Aid on all items in the left panel, from top down.

APFS won't work for Sierra or earlier, Sierra or earlier needs MacOS Extended Journaled. High Sierra won't work for APFS on earlier Macs.

Shift-Option-⌘-R

Install the macOS that came with your Mac, or the closest version still available.

2. Decide whether to erase (format) your disk

If you need to erase your disk before installing macOS, select Disk Utility from the Utilities window, then click Continue. You probably don't need to erase, unless you're selling or giving away your Mac or have an issue that requires you to erase. Learn more about when and how to erase.

3. Install macOS

After starting up from macOS Recovery, follow these steps to install macOS:

1. Choose Reinstall macOS (or Reinstall OS X) from the Utilities window.
2. Click Continue, then follow the onscreen instructions to choose your disk and begin installation.
3. If the installer asks to unlock your disk, enter the password you use to log in to your Mac. If it doesn't see your disk, or it says that it can't install on your computer or volume, you might need to erase your disk. 

1. Please allow installation to complete without putting your Mac to sleep or closing its lid. During installation, your Mac might restart and show a progress bar several times, and the screen might be empty for minutes at a time.

If your Mac restarts to a setup assistant, but you're selling or giving it away, press Command-Q to quit the assistant without completing setup. Then click Shut Down. When the new owner starts up the Mac, they can use their own information to complete setup.

If you never installed macOS Sierra 10.12.4

If macOS Sierra 10.12.4 or later was never installed on your Mac, macOS Recovery works differently:

• Command-R is still the recommended way to start up from macOS Recovery. This combination makes sure that the installation isn't associated with your Apple ID, which is important if you're selling or giving away your Mac.
• Option-Command-R installs the macOS that came with your Mac, or the closest version still available.
• Shift-Option-Command-R isn't available.

How to reinstall macOS - Apple Support 

Of course you may need to make a USB installer...

Create a bootable installer for macOS - Apple Support...

Create a bootable installer for macOS - Apple Support

Look into the Free LibreOffice, it handles MS Office files like Word & Excel, & Page Layout…

https://www.libreoffice.org/

Thanks to Matti Haveri…

MS Office for Mac Mini - Apple Community

LibreOffice 4.0.6.2_MacOS_PPC works in OS X 10.5.8 (last official PowerPC version).

LibreOffice 4.3.7.2_MacOS_x86.dmg (and x86-64?) works in OS X 10.6.

LibreOffice 7.0.4 works in macOS 10.11 and 10.14.

https://downloadarchive.documentfoundation.org/libreoffice/old/

Thanks to Servant to Cats…

LibreOffice is up to

• Version 7.5.4 – Requires Mojave or later
• Version 7.4.7 – Requires High Sierra or later
• The LibreOffice site is now offering
• Version 7.6.2 – Requires Catalina or later
• Version 7.5.7 – Requires Mojave or later

Release notes indicate that as of August 2023, new versions of LibreOffice will likely require Catalina or later.

If you click the link in that discussion, you will find that you can restore your Mac to a previous macOS version — if you had used Time Machine... but you wrote:

elsie222 wrote:

Before upgrading to Catalina I did not do a backup, ...

So... did you or didn't you?

NB: macOS versions subsequent to Catalina no longer back up macOS itself.

+1 for LibreOffice.