How to undo a save?
What do you mean?
Once you have closed a document you cannot carry out any actions on it. Just open it again.
First make sure to not save the bad version again before you did as described below.
If you enabled
'Tools' > 'Options' > 'Load/Save' > 'General' > 'Save' > 'Always create a backup copy',
you should find a file with the one previous version in the backup folder of your user profile.
It will have the known name, but the extension .bak instead of .odt (or respective).
Copy the file, paste it elsewhere, rename it for safety, and change the extension back to .odt.
Do with the document as you want.
This will only work if the other file wasn’t edited and saved again in-between.
Find the default location of the user profile..
Also:
To generally get access to more than one previous versions of a document you may use the “Timestamp Backup” extension originally created by Tibor Kovács, and available from github here.
I personally did not test it.