Undo
Click the Home | Undo | Undo command the 
button or press CTRL+Z on the keyboard to reverse the last operation performed. If the last operation cannot be reversed, the Undo command is grayed. After you close a file, the undo operations are cleared, so you cannot undo any operations performed before the file was closed. After you have undone an operation, the Redo command becomes available, allowing you to reverse the last completed Undo command.
Hover the cursor over the Undo command to see the last action. For example, after drawing a polyline the Undo command tooltip displays Undo Creation .
Zoom and pan operations are not included in the undo list. Consider using a view command or keyboard command to quickly move to a different view.
Grid Editor
Click the Grid Editor | Undo | Undo command or press CTRL+Z to reverse the last operation performed when in the grid editor view. Undo operations are only performed on ribbon commands in the Grid Editor . Property changes in the Grid Editor cannot be undone with the Undo command.
Undo Levels
Up to 100 undo levels can be set though Options dialog General page. If the Undo command is not available, check the Undo levels . If the value is set to zero, undo will not be available.