If you've come to here looking to undo a fix script, I have bad news for you. I cannot fix the problem you currently have, but I can show you how to save yourself in the future.

So the other day people were talking about what to use to fix some data in their instance. The choices were Background Scripts, and Fix Scripts.

So I took note about why would you ever use a background script in favor of a fix script. One thing that stood out was background scripts can "rollback" the updates.
Fix scripts don't show a way to do that. Then a few days later 0x111 shared on Discord that in fact they can be rolled back for fix scripts, but you have to set it up.

What do I mean? Well according to this hi article(KB0761248). You have to show the "Record for rollback" field, and check it if you intend to "rollback" the updates.

Here's what I saw

Feature Background Script Fix Script
Can choose your scope at execution :white_check_mark: :no_entry:
Can run on application install :no_entry: :white_check_mark:
Audit System Log only :white_check_mark:
Instant Return :white_check_mark: When run in foreground
Can run in the background :no_entry: :white_check_mark:
Syntax Highlighting :no_entry: :white_check_mark:
Intellisense :no_entry: :white_check_mark:
Versioning :no_entry: :white_check_mark:
Can be cancelled :white_check_mark: :white_check_mark:
Updates can be rolled back :white_check_mark: If you tell it

This feature is great, but why is it default unchecked? I know it's new as of London but come on. This makes using Fix Scripts the way to do things.

Further Reading

How long do these rollbacks last? Well that depends on the value of the property, glide.rollback.expiration_days. More about that here on this rollback contexts doc.

Thanks Kalai for the information on the rollback contexts.