Return the table to its state before the last action was taken and remove that action from the history.

To undo the last action:

In an open table or worksheet, click Actions > Undo Last Action.
The table is returned to its state before the last action was taken, and the action is removed from the history. You can repeat this all the way to the initial state of the table (when it was originally loaded into the current tab), if desired. Also, you can undo all actions at once by clicking Actions > Undo All Actions.
Note: This will have no effect if there are no prior actions (i.e., if no actions have yet been taken or if you have undone all of the steps in your history).
Note: Undoing the last action is different from going back one step. Undoing the last action removes that action from your history, so that you cannot subsequently redo that action.