Go to the property of the button.
Go to the "Other" tab.
Set "Cancel" to yes.

I'm afraid you can't cancel changes to the records in the subform easily.

Ops. I should have read your question closely. I missed the part of the subform.

Hi Maria, if you want to undo even after you leave the subform (and/or close the main form altogether), then you would need to create an "undo history" for the subform.  You would need to implement this on your own because Access doesn't have this capability built-in.  This would be a complicated job even for the best of Access developers because of the numerous things involved.  You would need to able to undo ALL EDITS (or it would be pointless to have an undo feature), and so you would need to save every single change that happens to a subform: new records added, existing records deleted or changed, field value changes, format changes in rich text fields (font name and size changes, etc.), etc.

An undo feature is mainly about undoing user entries.  But Access applications aren't always about user entries, since many of the changes to data may occur "behind the scenes," such as append and update queries executed with VBA code without the user knowing.  A good Access application tries to *minimize* user inputs, because manual data entry is prone to typos and other mistakes.  If user action is minimized, there is little need for an undo feature.  Word and Excel are mostly about manual data entries, so their undo/redo features are of greater use.