Delete a row, drag a column to another size, paste something, change an id in the panel: each is a step. Ctrl+Z (⌘+Z on a Mac) goes back one, and Ctrl+Y or Ctrl+Shift+Z comes forward again, as do the and buttons on the right of the toolbar. A change made after going back ends what could be redone.
While you are writing in a text or a field, the keys undo what you typed there, as usual; it becomes a step when you are done. The node you clicked last - the target, with a dashed outline - is the target again after a step, if it is still there.
Then undo it.
Delete this column, or the whole row.
One.
Two.
Three.