can i undo a mac update

There's no downgrade path for macOS, other than rolling in backups, or starting over.

Downgrading files may or may not work, as apps can update those files as part of the new macOS release or as part of associated app updates.

More generally, all of the old Adobe CS releases are no longer supported, and Adobe has migrated all customers over to subscriptions.

Your choice here is to pay for the Adobe CC subscription, or to lock in on an older macOS release and matching hardware until you're forced to migrate off or to upgrade your hardware and/or software, or to select a product other than Adobe.

There's no other way out of this while continuing to use Adobe software.

32 bit apps still run on Mohave. This is because those apps are so old now.

Restore from the back up you made before upgrading.

Can I undo the update?

Maybe. Read Revert your Mac to a previous macOS version.

That document begins with "if you use Time Machine to back up your Mac" which you certainly should have been doing, but even if you have not been using Time Machine follow it anyway.

Step 4 instructs you to select your Time Machine backup disk. If you do not have one, select the Mac's startup disk. It is usually named "Macintosh HD" unless you changed it. A dated list of available local snapshots and macOS versions will appear.

Choose the macOS version you wish to restore. Restoring in that manner takes only a minute or two.

I should add that downgrading macOS for the sole purpose of using outdated or obsolete software is far from an ideal solution.