Most actions in Elan can be undone via the (or CTRL+Z) menu. A redo (go back to the state before the undo action) is available to, via (or CTRL+Y) .
The following commands support undo/redo:
Add new tier / linguistic type
Copy tier / change parent tier
Insert annotation before / after
Change tier attributes / linguistic type
Clear selection (through the icon or keyboard shortcut)
Delete annotation / tier / linguistic type
Modify annotation / annotation time
Create new annotation
Copy / Paste / Duplicate annotation
Play around selection time setting
The tokenizer
Add / Change / Edit controlled vocabularies