To undo an action, simply tap the icon above the timeline. A notification will appear letting you know which action you've just undone. Need to redo an action? Simply tap the redo button.
You can undo or redo as often as you like within a single session, but once you return to the Project screen (either by saving or exporting), you won't be able to undo or redo.
Don't see it?
You must be on version 10.77 or newer.